raid-0 with mdadm vs lvm striping

raid-0 with mdadm vs lvm striping

[Henry, Andrew]

I'm thinking of buying a couple of disks to put in a striped array for performance.  I already have a raid-1 array with mdadm.

Are there are management or performance benefits of using mdadm raid-0 over using LVM striping?

Im thinking along the lines of "LVM might be a better choice because I can have my striping (by adding another disk to the LV) and also have the added bonus of it being more 'manageable' than raid, because I can grow or shrink the LV".

Is my idea full of holes?  Are there well known issues; performance wise or manageability wise of using an LVM as a striped array for performance?

Comments welcome
 --andrew

Re: raid-0 with mdadm vs lvm striping

[Drew]

> Are there are management or performance benefits of using mdadm raid-0 over using LVM striping?

I haven't used LVM striping so take my answer with a grain of salt. In
my reading on the web about the two I came to the conclusion that MD's
raid-0 code is a bit better tested then LVM's. For that reason, and
because I believe in one tool for each job, I went with MD to handle
the RAID.

> Im thinking along the lines of "LVM might be a better choice because I can have my striping (by adding another disk to the LV) and also have the added bonus of it being more 'manageable' than raid, because I can grow or shrink the LV".

If you run LVM on top of raid you can have both. :-) All my servers
run this config and it makes things easy to move around as needed, in
fact a couple of times it saved my data (and my bacon). And with newer
versions of mdadm, you can create some interesting raid configs that
LVM has no chance of creating.

So why striping? Aside from faster speeds, raid-0 is anything but redundant.


-- 
Drew

"Nothing in life is to be feared. It is only to be understood."
--Marie Curie

RE: raid-0 with mdadm vs lvm striping

[Henry, Andrew]

> If you run LVM on top of raid you can have both. :-) All my servers
> run this config and it makes things easy to move around as needed, in
> fact a couple of times it saved my data (and my bacon). And with newer
> versions of mdadm, you can create some interesting raid configs that
> LVM has no chance of creating.
> 
> So why striping? Aside from faster speeds, raid-0 is anything but redundant.
> --
> Drew

I need the speed, pure and simple.  It's going to be used as a scratch volume.  I already have a mirrored raid array with mdadm for my "real" data and backups, but I need something faster just to store temporary files on.

--andrew

Re: raid-0 with mdadm vs lvm striping

[Goswin von Brederlow]

"Henry, Andrew" <andrew.henry@logica.com> writes:

>> If you run LVM on top of raid you can have both. :-) All my servers
>> run this config and it makes things easy to move around as needed, in
>> fact a couple of times it saved my data (and my bacon). And with newer
>> versions of mdadm, you can create some interesting raid configs that
>> LVM has no chance of creating.
>> 
>> So why striping? Aside from faster speeds, raid-0 is anything but redundant.
>> --
>> Drew
>
> I need the speed, pure and simple.  It's going to be used as a scratch volume.  I already have a mirrored raid array with mdadm for my "real" data and backups, but I need something faster just to store temporary files on.
>
> --andrew

But why raid0 instead of striped lvm? Does that make a difference in
speed?

MfG
        Goswin

RE: raid-0 with mdadm vs lvm striping

[Henry, Andrew]

> 
> But why raid0 instead of striped lvm? Does that make a difference in
> speed?
> 
> MfG
>         Goswin

That's what my question was:  Will raid0 (striping) be faster or easier to manage than lvm striping

--andrew

Re: raid-0 with mdadm vs lvm striping

[Billy Crook]

On Wed, Aug 12, 2009 at 04:14, Henry, Andrew<andrew.henry@logica.com> wrote:
> That's what my question was:  Will raid0 (striping) be faster or easier to manage than lvm striping

LVM striping won't change performance for small files. (That is files,
smaller than the extent size.)  To be fair, RAID-0 striping won't
accelerate file access for files smaller than the stripe size either.
However, traditionally, raid0 stripe sizes are much much smaller than
LVM extent sizes.  Both are however adjustable.

Either way, you will be reading from one disk and then another, and
then another, etc.  The smaller the stripe size, the more likely
multiple disks will be doing their seeks and dma transfers at the same
time.  For this reason, I would recommend RAID-0 if you don't need
uptime, and you keep backups elsewhere, and all you need is the most
speed you can get.

LVM is more about flexibility than performance.  I don't use LVM
striping, but if I did, I would be using it to distribute wear between
multiple raid arrays; not for performance.

LVM on top of RAID is exceptionally common.  Use RAID for performance
and availability, then use LVM for flexibility.

"Easier to manage"?  Stop managing.  Start using.  With LVM on top of
raid, you can hot-move the logical volume off the raid0 array for
maintenance to the raid0 array.  You can extend it onto new raid0
arrays.  I'm fairly sure you can grow an existing raid0 array even
while it is in use onto additional disks.

But if you're playing with raid0 or lvm striping, I wouldn't as much
worry about being able to manage it while its in use.  Just do your
managing when you replace a failed disk (and have to recreate the
entire array or logical volume).
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Re: raid-0 with mdadm vs lvm striping

[Goswin von Brederlow]

Billy Crook <billycrook@gmail.com> writes:

> On Wed, Aug 12, 2009 at 04:14, Henry, Andrew<andrew.henry@logica.com> wrote:
>> That's what my question was:  Will raid0 (striping) be faster or easier to manage than lvm striping
>
> LVM striping won't change performance for small files. (That is files,
> smaller than the extent size.)  To be fair, RAID-0 striping won't
> accelerate file access for files smaller than the stripe size either.
> However, traditionally, raid0 stripe sizes are much much smaller than
> LVM extent sizes.  Both are however adjustable.

That sucks. LVM striping really should have a setting where it stripes
chunks within a PE.

MfG
        Goswin
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RE: raid-0 with mdadm vs lvm striping

[Guy Watkins]

} -----Original Message-----
} From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
} owner@vger.kernel.org] On Behalf Of Goswin von Brederlow
} Sent: Wednesday, August 12, 2009 10:09 PM
} To: Billy Crook
} Cc: Henry, Andrew; goswin-v-b@web.de; Drew; linux-raid@vger.kernel.org
} Subject: Re: raid-0 with mdadm vs lvm striping
} 
} Billy Crook <billycrook@gmail.com> writes:
} 
} > On Wed, Aug 12, 2009 at 04:14, Henry, Andrew<andrew.henry@logica.com>
} wrote:
} >> That's what my question was:  Will raid0 (striping) be faster or easier
} to manage than lvm striping
} >
} > LVM striping won't change performance for small files. (That is files,
} > smaller than the extent size.)  To be fair, RAID-0 striping won't
} > accelerate file access for files smaller than the stripe size either.
} > However, traditionally, raid0 stripe sizes are much much smaller than
} > LVM extent sizes.  Both are however adjustable.
} 
} That sucks. LVM striping really should have a setting where it stripes
} chunks within a PE.
} 
} MfG
}         Goswin

Why not use this option?

-I, --stripesize StripeSize
    Gives  the  number  of  kilobytes  for  the  granularity  of the
stripes.  StripeSize must be 2^n (n = 2 to 9) for metadata in LVM1 format.
For  metadata  in  LVM2  format, the stripe size may be a larger power of 2
but must not exceed the physical extent size.

I have never use lvcreate with Linux, but have with HP-UX.  Looks about the
same, but maybe I am confused.

Guy

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Re: raid-0 with mdadm vs lvm striping

[Goswin von Brederlow]

"Guy Watkins" <linux-raid@watkins-home.com> writes:

> } -----Original Message-----
> } From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
> } owner@vger.kernel.org] On Behalf Of Goswin von Brederlow
> } Sent: Wednesday, August 12, 2009 10:09 PM
> } To: Billy Crook
> } Cc: Henry, Andrew; goswin-v-b@web.de; Drew; linux-raid@vger.kernel.org
> } Subject: Re: raid-0 with mdadm vs lvm striping
> } 
> } Billy Crook <billycrook@gmail.com> writes:
> } 
> } > On Wed, Aug 12, 2009 at 04:14, Henry, Andrew<andrew.henry@logica.com>
> } wrote:
> } >> That's what my question was:  Will raid0 (striping) be faster or easier
> } to manage than lvm striping
> } >
> } > LVM striping won't change performance for small files. (That is files,
> } > smaller than the extent size.)  To be fair, RAID-0 striping won't
> } > accelerate file access for files smaller than the stripe size either.
> } > However, traditionally, raid0 stripe sizes are much much smaller than
> } > LVM extent sizes.  Both are however adjustable.
> } 
> } That sucks. LVM striping really should have a setting where it stripes
> } chunks within a PE.
> } 
> } MfG
> }         Goswin
>
> Why not use this option?

Ignorance. :) I never use raid0 so I never needed to look for all the
stripe parameters.

> -I, --stripesize StripeSize
>     Gives  the  number  of  kilobytes  for  the  granularity  of the
> stripes.  StripeSize must be 2^n (n = 2 to 9) for metadata in LVM1 format.
> For  metadata  in  LVM2  format, the stripe size may be a larger power of 2
> but must not exceed the physical extent size.
>
> I have never use lvcreate with Linux, but have with HP-UX.  Looks about the
> same, but maybe I am confused.
>
> Guy

MfG
        Goswin
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Re: raid-0 with mdadm vs lvm striping

[Keld Jørn Simonsen]

On Thu, Aug 13, 2009 at 04:09:10AM +0200, Goswin von Brederlow wrote:
> Billy Crook <billycrook@gmail.com> writes:
> 
> > On Wed, Aug 12, 2009 at 04:14, Henry, Andrew<andrew.henry@logica.com> wrote:
> >> That's what my question was:  Will raid0 (striping) be faster or easier to manage than lvm striping
> >
> > LVM striping won't change performance for small files. (That is files,
> > smaller than the extent size.)  To be fair, RAID-0 striping won't
> > accelerate file access for files smaller than the stripe size either.

Have you tested this? In theory, both LVM and raid0 striping should
accellerate access to small files substantially, for example if a
process accesses a number of small files, which are laid out in sequence
on the file system. This is a common case. 

I think this could come about eg if you roll in a tarball or package of
a system, then the small files are created in sequence, and the file
system should lay them out in sequence on the disk. When reading again,
the disk read ahead would secure that most of the data for a number of
small files were already read into the kernel cache when asked for, and
thus the files would be read at striping speeds. 

> > However, traditionally, raid0 stripe sizes are much much smaller than
> > LVM extent sizes.  Both are however adjustable.

What are the parameters? raid normally assumes 64 kiB chunks, but i
normally use 256 kiB chunks, which is the lowest chunk size that we are
recommending out of the linux-raid group. What are normal with LVM?

best regards
keld
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Re: raid-0 with mdadm vs lvm striping

[Drew]

> That's what my question was:  Will raid0 (striping) be faster or easier to manage than lvm striping

I don't know about faster but IMO it'll be easier with raid0. With LVM
you have to specify striping *every* time you create a new LV. With an
underlying raid0 array as the sole physical volume, you can create the
LV knowing it's striped.

One less switch to forget.


-- 
Drew

"Nothing in life is to be feared. It is only to be understood."
--Marie Curie
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