raid10 layout for 2xSSDs

raid10 layout for 2xSSDs

[Kasper Sandberg]

Hello.

I've been wanting to create a raid10 array of two SSDs, and I am
currently considering the layout.

As i understand it, near layout is similar to raid1, and will only
provide a speedup if theres 2 reads at the same time, not a single
sequential read.

so the choice is really between far and offset. As i see it, the
difference is, that offset tries to reduce the seeking for writing
compared to far, but that if you dont consider the seeking penalty,
average sequential write speed across the entire array should be roughly
the same with offset and far, with offset perhaps being a tad more
"stable", is this a correct assumption? if it is, that would mean offset
provides a higher "garantueed" speed than far, but with a lower maximum
speed.

mvh.
Kasper Sandberg

Re: raid10 layout for 2xSSDs

[Goswin von Brederlow]

Kasper Sandberg <postmaster@metanurb.dk> writes:

> Hello.
>
> I've been wanting to create a raid10 array of two SSDs, and I am
> currently considering the layout.
>
> As i understand it, near layout is similar to raid1, and will only
> provide a speedup if theres 2 reads at the same time, not a single
> sequential read.
>
> so the choice is really between far and offset. As i see it, the
> difference is, that offset tries to reduce the seeking for writing
> compared to far, but that if you dont consider the seeking penalty,
> average sequential write speed across the entire array should be roughly
> the same with offset and far, with offset perhaps being a tad more
> "stable", is this a correct assumption? if it is, that would mean offset
> provides a higher "garantueed" speed than far, but with a lower maximum
> speed.
>
> mvh.
> Kasper Sandberg

Doesn't offset have the copies of each stripe right next to each other
(just rotated). So writing one stripe would actualy write a 2 block
continous chunk per device.

With far copies the stripes are far from each other and you get 2
seperate continious chunks per device.

What I'm aiming at is that offset might better fit into erase blocks,
cause less internal fragmentation on the disk and give better wear
leveling. Might improve speed and lifetime. But that is just a
thought. Maybe test and do ask Intel (or other vendors) about it.

MfG
        Goswin

Re: raid10 layout for 2xSSDs

[Keld Jørn Simonsen]

On Mon, Nov 16, 2009 at 04:26:32PM +0100, Goswin von Brederlow wrote:
> Kasper Sandberg <postmaster@metanurb.dk> writes:
> 
> > Hello.
> >
> > I've been wanting to create a raid10 array of two SSDs, and I am
> > currently considering the layout.
> >
> > As i understand it, near layout is similar to raid1, and will only
> > provide a speedup if theres 2 reads at the same time, not a single
> > sequential read.
> >
> > so the choice is really between far and offset. As i see it, the
> > difference is, that offset tries to reduce the seeking for writing
> > compared to far, but that if you dont consider the seeking penalty,
> > average sequential write speed across the entire array should be roughly
> > the same with offset and far, with offset perhaps being a tad more
> > "stable", is this a correct assumption? if it is, that would mean offset
> > provides a higher "garantueed" speed than far, but with a lower maximum
> > speed.
> >
> > mvh.
> > Kasper Sandberg
> 
> Doesn't offset have the copies of each stripe right next to each other
> (just rotated). So writing one stripe would actualy write a 2 block
> continous chunk per device.
> 
> With far copies the stripes are far from each other and you get 2
> seperate continious chunks per device.
> 
> What I'm aiming at is that offset might better fit into erase blocks,
> cause less internal fragmentation on the disk and give better wear
> leveling. Might improve speed and lifetime. But that is just a
> thought. Maybe test and do ask Intel (or other vendors) about it.

I think the caching of the file system levies out all of this, if we
talk SSD. The presumption on this is that there is no rotational latency
with SSD, and that no head movement. 

The caching means that for writing, more buffers are chained together
and can be written at once. For near, logical blocks 1-8
can be written to sector 0 of disk 1 in one go, and logical blocks
1-8 can be written to sector 0 of disk 2 in one go.
For far it will be for disk 1: block 1, 3, 5, and 7 to sector 0, and
block 2, 4, 6 and 8 to sector n/2 - n being the number of sectors on the
diskpartition. For far and disk 2, it will be blocks 2, 4, 6 and 8 to
sector 0, and blocks 1, 3, 5 and 7 to sector n/2. caching thus reduces
seeking significantly, from once per block, to once per flushing of the
cache (syncing). Similarily the cache also would almost eliminate
seeking for the offset layout.

but I would like to see some numbers on this, for SSD.
Why don't you try it out and tell us what you find?

Best regards
keld

Re: raid10 layout for 2xSSDs

[Goswin von Brederlow]

Keld Jørn Simonsen <keld@keldix.com> writes:

> On Mon, Nov 16, 2009 at 04:26:32PM +0100, Goswin von Brederlow wrote:
>> Kasper Sandberg <postmaster@metanurb.dk> writes:
>> 
>> > Hello.
>> >
>> > I've been wanting to create a raid10 array of two SSDs, and I am
>> > currently considering the layout.
>> >
>> > As i understand it, near layout is similar to raid1, and will only
>> > provide a speedup if theres 2 reads at the same time, not a single
>> > sequential read.
>> >
>> > so the choice is really between far and offset. As i see it, the
>> > difference is, that offset tries to reduce the seeking for writing
>> > compared to far, but that if you dont consider the seeking penalty,
>> > average sequential write speed across the entire array should be roughly
>> > the same with offset and far, with offset perhaps being a tad more
>> > "stable", is this a correct assumption? if it is, that would mean offset
>> > provides a higher "garantueed" speed than far, but with a lower maximum
>> > speed.
>> >
>> > mvh.
>> > Kasper Sandberg
>> 
>> Doesn't offset have the copies of each stripe right next to each other
>> (just rotated). So writing one stripe would actualy write a 2 block
>> continous chunk per device.
>> 
>> With far copies the stripes are far from each other and you get 2
>> seperate continious chunks per device.
>> 
>> What I'm aiming at is that offset might better fit into erase blocks,
>> cause less internal fragmentation on the disk and give better wear
>> leveling. Might improve speed and lifetime. But that is just a
>> thought. Maybe test and do ask Intel (or other vendors) about it.
>
> I think the caching of the file system levies out all of this, if we
> talk SSD. The presumption on this is that there is no rotational latency
> with SSD, and that no head movement. 

Filesystem has nothing to do with this. It caches the same for both
situations. The only change happens on the block layer.

> The caching means that for writing, more buffers are chained together
> and can be written at once. For near, logical blocks 1-8
> can be written to sector 0 of disk 1 in one go, and logical blocks
> 1-8 can be written to sector 0 of disk 2 in one go.

Which is what I was saying.

> For far it will be for disk 1: block 1, 3, 5, and 7 to sector 0, and
> block 2, 4, 6 and 8 to sector n/2 - n being the number of sectors on the
> diskpartition. For far and disk 2, it will be blocks 2, 4, 6 and 8 to
> sector 0, and blocks 1, 3, 5 and 7 to sector n/2. caching thus reduces
> seeking significantly, from once per block, to once per flushing of the
> cache (syncing). Similarily the cache also would almost eliminate
> seeking for the offset layout.

There is no seeking (head movement) and no rotational latency
involved. That part is completly irelevant.

The important part is that you now have 4 IO operations of half the
size comapred to the 2 IO operations of the offset case. The speed and
wear will depends on the quality of the SSD, how well it copes with
small IO.

> but I would like to see some numbers on this, for SSD.
> Why don't you try it out and tell us what you find?

I would be interested in this myself. I don't have an SSD yet but I'm
tempted to buy. When you test please also test random access. I would
guess that in any sequential test the amount of caching going on will
make all IO operations so big that no difference shows.

> Best regards
> keld

MfG
        Goswin
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Re: raid10 layout for 2xSSDs

[Kasper Sandberg]

On Tue, 2009-11-17 at 05:34 +0100, Goswin von Brederlow wrote:
> Keld Jørn Simonsen <keld@keldix.com> writes:
> 
> > On Mon, Nov 16, 2009 at 04:26:32PM +0100, Goswin von Brederlow wrote:
> >> Kasper Sandberg <postmaster@metanurb.dk> writes:
> >> 
> >> > Hello.
> >> >
> >> > I've been wanting to create a raid10 array of two SSDs, and I am
> >> > currently considering the layout.
> >> >
> >> > As i understand it, near layout is similar to raid1, and will only
> >> > provide a speedup if theres 2 reads at the same time, not a single
> >> > sequential read.
> >> >
> >> > so the choice is really between far and offset. As i see it, the
> >> > difference is, that offset tries to reduce the seeking for writing
> >> > compared to far, but that if you dont consider the seeking penalty,
> >> > average sequential write speed across the entire array should be roughly
> >> > the same with offset and far, with offset perhaps being a tad more
> >> > "stable", is this a correct assumption? if it is, that would mean offset
> >> > provides a higher "garantueed" speed than far, but with a lower maximum
> >> > speed.
> >> >
> >> > mvh.
> >> > Kasper Sandberg
> >> 
> >> Doesn't offset have the copies of each stripe right next to each other
> >> (just rotated). So writing one stripe would actualy write a 2 block
> >> continous chunk per device.
> >> 
> >> With far copies the stripes are far from each other and you get 2
> >> seperate continious chunks per device.
> >> 
> >> What I'm aiming at is that offset might better fit into erase blocks,
> >> cause less internal fragmentation on the disk and give better wear
> >> leveling. Might improve speed and lifetime. But that is just a
> >> thought. Maybe test and do ask Intel (or other vendors) about it.
> >
> > I think the caching of the file system levies out all of this, if we
> > talk SSD. The presumption on this is that there is no rotational latency
> > with SSD, and that no head movement. 
> 
> Filesystem has nothing to do with this. It caches the same for both
> situations. The only change happens on the block layer.
> 
> > The caching means that for writing, more buffers are chained together
> > and can be written at once. For near, logical blocks 1-8
> > can be written to sector 0 of disk 1 in one go, and logical blocks
> > 1-8 can be written to sector 0 of disk 2 in one go.
> 
> Which is what I was saying.
> 
> > For far it will be for disk 1: block 1, 3, 5, and 7 to sector 0, and
> > block 2, 4, 6 and 8 to sector n/2 - n being the number of sectors on the
> > diskpartition. For far and disk 2, it will be blocks 2, 4, 6 and 8 to
> > sector 0, and blocks 1, 3, 5 and 7 to sector n/2. caching thus reduces
> > seeking significantly, from once per block, to once per flushing of the
> > cache (syncing). Similarily the cache also would almost eliminate
> > seeking for the offset layout.
> 
> There is no seeking (head movement) and no rotational latency
> involved. That part is completly irelevant.
> 
> The important part is that you now have 4 IO operations of half the
> size comapred to the 2 IO operations of the offset case. The speed and
> wear will depends on the quality of the SSD, how well it copes with
> small IO.
Very interresting, i have some older SSDs where they are slower when
doing a SMALLER write, so in this case offset should be alot better.

> 
> > but I would like to see some numbers on this, for SSD.
> > Why don't you try it out and tell us what you find?
> 
> I would be interested in this myself. I don't have an SSD yet but I'm
> tempted to buy. When you test please also test random access. I would
> guess that in any sequential test the amount of caching going on will
> make all IO operations so big that no difference shows.
> 
> > Best regards
> > keld
> 
> MfG
>         Goswin
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Re: raid10 layout for 2xSSDs

[Robin Hill]

[-- Attachment #1: Type: text/plain, Size: 1229 bytes --]

On Mon Nov 16, 2009 at 04:26:32PM +0100, Goswin von Brederlow wrote:

> What I'm aiming at is that offset might better fit into erase blocks,
> cause less internal fragmentation on the disk and give better wear
> leveling. Might improve speed and lifetime. But that is just a
> thought. Maybe test and do ask Intel (or other vendors) about it.
> 
I very much doubt this will make any difference.  With SSDs you have to
throw out any preconceptions of internal layout you may have.  You have
absolutely no idea (or control of) where two consecutive blocks will
actually get written.  Fragmentation and seek time are thus irrelevant
(or uncontrollable anyway).

I don't see how any RAID-10 layout would perform better than another
with SSDs, unless there's internal optimisations/constraints which
affect sequential reading from multiple devices.  I'm not aware of any
though - RAID-10 n2 may be the same layout as RAID-1 but it's an
entirely separate piece of code.

Cheers,
    Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
   / / )      | Little Jim says ....                            |
  // !!       |      "He fallen in de water !!"                 |

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Re: raid10 layout for 2xSSDs

[Christopher Chen]

On Mon, Nov 16, 2009 at 8:31 AM, Robin Hill <robin@robinhill.me.uk> wrote:
> On Mon Nov 16, 2009 at 04:26:32PM +0100, Goswin von Brederlow wrote:
>
>> What I'm aiming at is that offset might better fit into erase blocks,
>> cause less internal fragmentation on the disk and give better wear
>> leveling. Might improve speed and lifetime. But that is just a
>> thought. Maybe test and do ask Intel (or other vendors) about it.
>>
> I very much doubt this will make any difference.  With SSDs you have to
> throw out any preconceptions of internal layout you may have.  You have
> absolutely no idea (or control of) where two consecutive blocks will
> actually get written.  Fragmentation and seek time are thus irrelevant
> (or uncontrollable anyway).
>
> I don't see how any RAID-10 layout would perform better than another
> with SSDs, unless there's internal optimisations/constraints which
> affect sequential reading from multiple devices.  I'm not aware of any
> though - RAID-10 n2 may be the same layout as RAID-1 but it's an
> entirely separate piece of code.

Don't forget that RAID-1 also does balanced reads.

-- 
Chris Chen <muffaleta@gmail.com>
"The fact that yours is better than anyone else's
is not a guarantee that it's any good."
-- Seen on a wall
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Re: raid10 layout for 2xSSDs

[Robin Hill]

[-- Attachment #1: Type: text/plain, Size: 1643 bytes --]

On Mon Nov 16, 2009 at 08:38:34AM -0800, Christopher Chen wrote:

> On Mon, Nov 16, 2009 at 8:31 AM, Robin Hill <robin@robinhill.me.uk> wrote:
> > On Mon Nov 16, 2009 at 04:26:32PM +0100, Goswin von Brederlow wrote:
> >
> >> What I'm aiming at is that offset might better fit into erase blocks,
> >> cause less internal fragmentation on the disk and give better wear
> >> leveling. Might improve speed and lifetime. But that is just a
> >> thought. Maybe test and do ask Intel (or other vendors) about it.
> >>
> > I very much doubt this will make any difference.  With SSDs you have to
> > throw out any preconceptions of internal layout you may have.  You have
> > absolutely no idea (or control of) where two consecutive blocks will
> > actually get written.  Fragmentation and seek time are thus irrelevant
> > (or uncontrollable anyway).
> >
> > I don't see how any RAID-10 layout would perform better than another
> > with SSDs, unless there's internal optimisations/constraints which
> > affect sequential reading from multiple devices.  I'm not aware of any
> > though - RAID-10 n2 may be the same layout as RAID-1 but it's an
> > entirely separate piece of code.
> 
> Don't forget that RAID-1 also does balanced reads.
> 
Only for parallel reads.  A single sequential read will only access a
single disk, whereas I believe for RAID-10 it will access both disks.

Cheers,
    Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
   / / )      | Little Jim says ....                            |
  // !!       |      "He fallen in de water !!"                 |

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Re: raid10 layout for 2xSSDs

[Goswin von Brederlow]

Robin Hill <robin@robinhill.me.uk> writes:

> On Mon Nov 16, 2009 at 04:26:32PM +0100, Goswin von Brederlow wrote:
>
>> What I'm aiming at is that offset might better fit into erase blocks,
>> cause less internal fragmentation on the disk and give better wear
>> leveling. Might improve speed and lifetime. But that is just a
>> thought. Maybe test and do ask Intel (or other vendors) about it.
>> 
> I very much doubt this will make any difference.  With SSDs you have to
> throw out any preconceptions of internal layout you may have.  You have
> absolutely no idea (or control of) where two consecutive blocks will
> actually get written.  Fragmentation and seek time are thus irrelevant
> (or uncontrollable anyway).
>
> I don't see how any RAID-10 layout would perform better than another
> with SSDs, unless there's internal optimisations/constraints which
> affect sequential reading from multiple devices.  I'm not aware of any
> though - RAID-10 n2 may be the same layout as RAID-1 but it's an
> entirely separate piece of code.
>
> Cheers,
>     Robin

Depending on the SSD in question the limiting factor will be the
number of IO operations per second. Some SSDs have shown that they can
write the same number of 1 byte blocks per second as they can write
64k bytes per second. If offset writes 2x 64k but far writes 4x 32k
then far will be half the speed on such a cheap SSD.

MfG
        Goswin

Re: raid10 layout for 2xSSDs

[Christopher Chen]

On Mon, Nov 16, 2009 at 6:29 AM, Kasper Sandberg <postmaster@metanurb.dk> wrote:
> Hello.
>
> I've been wanting to create a raid10 array of two SSDs, and I am
> currently considering the layout.
>
> As i understand it, near layout is similar to raid1, and will only
> provide a speedup if theres 2 reads at the same time, not a single
> sequential read.
>
> so the choice is really between far and offset. As i see it, the
> difference is, that offset tries to reduce the seeking for writing
> compared to far, but that if you dont consider the seeking penalty,
> average sequential write speed across the entire array should be roughly
> the same with offset and far, with offset perhaps being a tad more
> "stable", is this a correct assumption? if it is, that would mean offset
> provides a higher "garantueed" speed than far, but with a lower maximum
> speed.

Do you plan to have more than two devices in the array? Raid 10 isn't
magic. If you don't have more than do devices, I suppose your seek
time might be half for reads (and higher for writes), but you won't be
able to do any striping.

I'm a bit confused as to the number of people popping in recently
wanting to run raid 10 on two disk "arrays".

cc

-- 
Chris Chen <muffaleta@gmail.com>
"The fact that yours is better than anyone else's
is not a guarantee that it's any good."
-- Seen on a wall

Re: raid10 layout for 2xSSDs

[Kasper Sandberg]

On Mon, 2009-11-16 at 08:08 -0800, Christopher Chen wrote:
> On Mon, Nov 16, 2009 at 6:29 AM, Kasper Sandberg <postmaster@metanurb.dk> wrote:
> > Hello.
> >
> > I've been wanting to create a raid10 array of two SSDs, and I am
> > currently considering the layout.
> >
> > As i understand it, near layout is similar to raid1, and will only
> > provide a speedup if theres 2 reads at the same time, not a single
> > sequential read.
> >
> > so the choice is really between far and offset. As i see it, the
> > difference is, that offset tries to reduce the seeking for writing
> > compared to far, but that if you dont consider the seeking penalty,
> > average sequential write speed across the entire array should be roughly
> > the same with offset and far, with offset perhaps being a tad more
> > "stable", is this a correct assumption? if it is, that would mean offset
> > provides a higher "garantueed" speed than far, but with a lower maximum
> > speed.
> 
> Do you plan to have more than two devices in the array? Raid 10 isn't
no
> magic. If you don't have more than do devices, I suppose your seek
> time might be half for reads (and higher for writes), but you won't be
> able to do any striping.
> 
> I'm a bit confused as to the number of people popping in recently
> wanting to run raid 10 on two disk "arrays".
to get the doubled singlestream sequential read performance..
> 
> cc
> 

Re: raid10 layout for 2xSSDs

[Majed B.]

Wouldn't you be getting the same data twice? or would the kernel
request the half the data from one disk & the other from the other
disk? (file1-> chunk1@disk1 & chunk2@disk2, chunk3@disk1 &
chunk3@disk2)

On Tue, Nov 17, 2009 at 12:02 AM, Kasper Sandberg
<postmaster@metanurb.dk> wrote:
>
> On Mon, 2009-11-16 at 08:08 -0800, Christopher Chen wrote:
> > On Mon, Nov 16, 2009 at 6:29 AM, Kasper Sandberg <postmaster@metanurb.dk> wrote:
> > > Hello.
> > >
> > > I've been wanting to create a raid10 array of two SSDs, and I am
> > > currently considering the layout.
> > >
> > > As i understand it, near layout is similar to raid1, and will only
> > > provide a speedup if theres 2 reads at the same time, not a single
> > > sequential read.
> > >
> > > so the choice is really between far and offset. As i see it, the
> > > difference is, that offset tries to reduce the seeking for writing
> > > compared to far, but that if you dont consider the seeking penalty,
> > > average sequential write speed across the entire array should be roughly
> > > the same with offset and far, with offset perhaps being a tad more
> > > "stable", is this a correct assumption? if it is, that would mean offset
> > > provides a higher "garantueed" speed than far, but with a lower maximum
> > > speed.
> >
> > Do you plan to have more than two devices in the array? Raid 10 isn't
> no
> > magic. If you don't have more than do devices, I suppose your seek
> > time might be half for reads (and higher for writes), but you won't be
> > able to do any striping.
> >
> > I'm a bit confused as to the number of people popping in recently
> > wanting to run raid 10 on two disk "arrays".
> to get the doubled singlestream sequential read performance..
> >
> > cc
> >
>
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> the body of a message to majordomo@vger.kernel.org
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--
      Majed B.
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Re: raid10 layout for 2xSSDs

[Kasper Sandberg]

On Tue, 2009-11-17 at 00:19 +0300, Majed B. wrote:
> Wouldn't you be getting the same data twice? or would the kernel
> request the half the data from one disk & the other from the other
> disk? (file1-> chunk1@disk1 & chunk2@disk2, chunk3@disk1 &
> chunk3@disk2)

it will request different chunks from different disks, to speed up

> 
> On Tue, Nov 17, 2009 at 12:02 AM, Kasper Sandberg
> <postmaster@metanurb.dk> wrote:
> >
> > On Mon, 2009-11-16 at 08:08 -0800, Christopher Chen wrote:
> > > On Mon, Nov 16, 2009 at 6:29 AM, Kasper Sandberg <postmaster@metanurb.dk> wrote:
> > > > Hello.
> > > >
> > > > I've been wanting to create a raid10 array of two SSDs, and I am
> > > > currently considering the layout.
> > > >
> > > > As i understand it, near layout is similar to raid1, and will only
> > > > provide a speedup if theres 2 reads at the same time, not a single
> > > > sequential read.
> > > >
> > > > so the choice is really between far and offset. As i see it, the
> > > > difference is, that offset tries to reduce the seeking for writing
> > > > compared to far, but that if you dont consider the seeking penalty,
> > > > average sequential write speed across the entire array should be roughly
> > > > the same with offset and far, with offset perhaps being a tad more
> > > > "stable", is this a correct assumption? if it is, that would mean offset
> > > > provides a higher "garantueed" speed than far, but with a lower maximum
> > > > speed.
> > >
> > > Do you plan to have more than two devices in the array? Raid 10 isn't
> > no
> > > magic. If you don't have more than do devices, I suppose your seek
> > > time might be half for reads (and higher for writes), but you won't be
> > > able to do any striping.
> > >
> > > I'm a bit confused as to the number of people popping in recently
> > > wanting to run raid 10 on two disk "arrays".
> > to get the doubled singlestream sequential read performance..
> > >
> > > cc
> > >
> >
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
> 
> 
> --
>       Majed B.
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Re: raid10 layout for 2xSSDs

[Goswin von Brederlow]

Christopher Chen <muffaleta@gmail.com> writes:

> On Mon, Nov 16, 2009 at 6:29 AM, Kasper Sandberg <postmaster@metanurb.dk> wrote:
>> Hello.
>>
>> I've been wanting to create a raid10 array of two SSDs, and I am
>> currently considering the layout.
>>
>> As i understand it, near layout is similar to raid1, and will only
>> provide a speedup if theres 2 reads at the same time, not a single
>> sequential read.
>>
>> so the choice is really between far and offset. As i see it, the
>> difference is, that offset tries to reduce the seeking for writing
>> compared to far, but that if you dont consider the seeking penalty,
>> average sequential write speed across the entire array should be roughly
>> the same with offset and far, with offset perhaps being a tad more
>> "stable", is this a correct assumption? if it is, that would mean offset
>> provides a higher "garantueed" speed than far, but with a lower maximum
>> speed.
>
> Do you plan to have more than two devices in the array? Raid 10 isn't
> magic. If you don't have more than do devices, I suppose your seek
> time might be half for reads (and higher for writes), but you won't be
> able to do any striping.
>
> I'm a bit confused as to the number of people popping in recently
> wanting to run raid 10 on two disk "arrays".
>
> cc

I think you are missing the fact that linux has a special raid10
module. This is not just a raid0 over raid1 (or raid1 over raid0).
The raid10 module is much more flexible and allows to have X copies of
the data on Y disks (for any X <= Y) in different layouts.

The layouts are like this (for 2 copies on 2 disks):

near (same as raid1):

Disk A: 0 1 2 3 4 5 6 7 8 9
Disk B: 0 1 2 3 4 5 6 7 8 9 

offset:

Disk A: 0 1 2 3 4 5 6 7 8 9
Disk B: 1 0 3 2 5 4 7 6 9 8

far:

Disk A: 0 2 4 6 8 1 3 5 7 9
Disk B: 1 3 5 7 9 0 2 4 6 8

In the case of offset and far copies you can see that the data is
striped like in raid0 and the raid10 module will read data from
multiple drives in parallel even with a single stream.

In raid10 far mode the read will also (afaik, not 100% sure) always
come from the first half of the disk. With rotational disks that is
usualy the faster part. So you not only get double the read speed from
striping but also more speed from the disks itself. For example my
disk does 80MB/s at the start, 60MB/s in the middle and 40MB/s at the
end. Reads will only use the 80-60MB/s range now. Jupey.

MfG
        Goswin