Best size partition for a mdadm array

Best size partition for a mdadm array

[Andrea Bolandrina]

Hi All,

In choosing the right size for a mdadm array the manual sais:
"Sometimes a replacement drive can be a little smaller than the
original drives though this should be minimised by IDEMA standards.
Such a replacement drive will be rejected by md.
To guard against this it can be useful to set the initial size
slightly smaller than the smaller device with the aim that it will
still be larger than any replacement."

I want to do a 5x2TB md (raid 6) array .
Could you advise me how "slightly smaller" should my partitions be?

100MB smaller than the actual size? 1GB?

Thank in advance for the attention.

Regards,
Andrea

Re: Best size partition for a mdadm array

[Tudor Holton]

Hi Andrea,

I noticed your question wasn't answered, so I'll give it a shot. Bear in 
mind that I'm relatively new here, so take everything I say with a pinch 
of salt. :-)

At some point in the past, manufacturers of hard disks released disks 
with the same "marketing" size but different actual capacities.  For 
example, instead of 1TB being a 1 Tebibyte 2^40 (1,099,511,627,776 
bytes), they sold lower capacity drives by using a different base.   1TB 
could be 1000GB (10^3*2^30 = 1,073,741,824,000 bytes), 1024GB (2^10*10^9 
= 1,024,000,000,000 bytes) or even a "true" Terabyte at 10^12 bytes 
(1,000,000,000,000 bytes).  If we were swapping brands, and moved from a 
2^40 to a 10^12 model, then our array wouldn't fit anymore. :-(

These days, things are a bit clearer as people understand the 
differences between the Terabytes and Tebibytes.

Thankfully, most modern disk sizes are standardised, but still with some 
small variances.  It would be wise to look at the drives you have 
purchased and look at the actual storage capacity then take the lowest 
one and use that.  You may want to add in some additional buffer, also, 
just in case.

I hope that helps you make your decision. :-)

Cheers,
Tudor.

On 24/05/13 19:25, Andrea Bolandrina wrote:
> Hi All,
>
> In choosing the right size for a mdadm array the manual sais:
> "Sometimes a replacement drive can be a little smaller than the
> original drives though this should be minimised by IDEMA standards.
> Such a replacement drive will be rejected by md.
> To guard against this it can be useful to set the initial size
> slightly smaller than the smaller device with the aim that it will
> still be larger than any replacement."
>
> I want to do a 5x2TB md (raid 6) array .
> Could you advise me how "slightly smaller" should my partitions be?
>
> 100MB smaller than the actual size? 1GB?
>
> Thank in advance for the attention.
>
> Regards,
> Andrea
> --
> 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

Re: Best size partition for a mdadm array

[Andrea Bolandrina]

Hi Tudor.

All disks I have report the same size, even though they are different
brand/model
#fdisk -l | grep Disk
Disk /dev/sdc: 2000.4 GB, 2000398934016 bytes
Disk /dev/sdd: 2000.4 GB, 2000398934016 bytes
Disk /dev/sde: 2000.4 GB, 2000398934016 bytes
Disk /dev/sdf: 2000.4 GB, 2000398934016 bytes
Disk /dev/sdg: 2000.4 GB, 2000398934016 bytes

They are 2 WD RED (WDC WD20EFRX-68AX9N0),
1 WD GREEN (WDC WD20EARX-00PASB0) and
2 SAMSUNG (HD204UI).

If the 2 WD RED bahave as they should I might buy some more as soon as
the others fail,
otherwise I will try something different (that's why I want to keep
the system as flexible as possible).

The 2 samsung SMART infos are already "screaming for help" and,
actually, a third Samsung already failed last year.
Terrible drives, terrible support from Samsung that replaced the drive
(still under warranty) with a refurbished one that failed within 10
days from arrival.

Anyway, regarding how much free space to leave, another user suggested
2-10 MB should be sufficient.

Thanks for your infos.
Andrea

On Mon, May 27, 2013 at 4:36 AM, Tudor Holton <tudor@smartguide.com.au> wrote:
> Hi Andrea,
>
> I noticed your question wasn't answered, so I'll give it a shot. Bear in
> mind that I'm relatively new here, so take everything I say with a pinch
> of
> salt. :-)
>
> At some point in the past, manufacturers of hard disks released disks with
> the same "marketing" size but different actual capacities.  For example,
> instead of 1TB being a 1 Tebibyte 2^40 (1,099,511,627,776 bytes), they
> sold
> lower capacity drives by using a different base.   1TB could be 1000GB
> (10^3*2^30 = 1,073,741,824,000 bytes), 1024GB (2^10*10^9 =
> 1,024,000,000,000
> bytes) or even a "true" Terabyte at 10^12 bytes (1,000,000,000,000 bytes).
> If we were swapping brands, and moved from a 2^40 to a 10^12 model, then
> our
> array wouldn't fit anymore. :-(
>
> These days, things are a bit clearer as people understand the differences
> between the Terabytes and Tebibytes.
>
> Thankfully, most modern disk sizes are standardised, but still with some
> small variances.  It would be wise to look at the drives you have
> purchased
> and look at the actual storage capacity then take the lowest one and use
> that.  You may want to add in some additional buffer, also, just in case.
>
> I hope that helps you make your decision. :-)
>
> Cheers,
> Tudor.
>
>
> On 24/05/13 19:25, Andrea Bolandrina wrote:
>>
>> Hi All,
>>
>> In choosing the right size for a mdadm array the manual sais:
>> "Sometimes a replacement drive can be a little smaller than the
>> original drives though this should be minimised by IDEMA standards.
>> Such a replacement drive will be rejected by md.
>> To guard against this it can be useful to set the initial size
>> slightly smaller than the smaller device with the aim that it will
>> still be larger than any replacement."
>>
>> I want to do a 5x2TB md (raid 6) array .
>> Could you advise me how "slightly smaller" should my partitions be?
>>
>> 100MB smaller than the actual size? 1GB?
>>
>> Thank in advance for the attention.
>>
>> Regards,
>> Andrea
>> --
>> 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
>
>

Re: Best size partition for a mdadm array

[Adam Goryachev]

On Mon, May 27, 2013 18:38, Andrea Bolandrina wrote:
> Hi Tudor.
>
> All disks I have report the same size, even though they are different
> brand/model

Discussion on this list in the past would suggest the manufacturers have
agreed to make equal sized drives in future, so this shouldn't really be
an issue for 2TB drives or larger.

> If the 2 WD RED bahave as they should I might buy some more as soon as
> the others fail,
> otherwise I will try something different (that's why I want to keep
> the system as flexible as possible).

Why not just put a 2G swap partition at the end of each drive, if needed,
you can reduce the swap partition on the new drive if it is a bit smaller
than the rest, you have up to 2G of flexibility.... (or 1G etc..) This
means the space isn't wasted overall, but there is no issue if you need to
make it a bit smaller/larger/etc...

> The 2 samsung SMART infos are already "screaming for help" and,
> actually, a third Samsung already failed last year.
> Terrible drives, terrible support from Samsung that replaced the drive
> (still under warranty) with a refurbished one that failed within 10
> days from arrival.

Personally, apart from a "batch" of WD Caviar Black drives that all failed
within a week, of had really good experience with them, and also the WD
RE4 drives.... I haven't yet tried the red, but would be interested to
hear how you go.

Regards,
Adam

Re: Best size partition for a mdadm array

[Roman Mamedov]

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On Mon, 27 May 2013 19:11:32 +1000
"Adam Goryachev" <mailinglists@websitemanagers.com.au> wrote:

> On Mon, May 27, 2013 18:38, Andrea Bolandrina wrote:
> > Hi Tudor.
> >
> > All disks I have report the same size, even though they are different
> > brand/model
> 
> Discussion on this list in the past would suggest the manufacturers have
> agreed to make equal sized drives in future, so this shouldn't really be
> an issue for 2TB drives or larger.

There are also some overly smart BIOSes, which in certain motherboard models
and versions are configured to automatically store a backup of the BIOS on the
primary HDD. What they do is cut off a small area from the HDD's "tail" by
establishing an HPA there, and put the BIOS copy on that area, possibly
overwriting whatever was there previously.

http://forums.overclockers.com.au/showthread.php?t=885212
http://matt.simerson.net/news/2009/07/22/hpa-host-protected-area
http://www.gigabyte.com/webpage/8/article_04_bios_explained.htm

The forum discussion linked cites the actual number to be 2113 sectors which is
about 1 MB.

Personally I always leave about 8 MB in the end of all disks just in case.


-- 
With respect,
Roman

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Re: Best size partition for a mdadm array

[Andrea Bolandrina]

The OS, as well as the swap partition, are on an SSD drive, therefore
using the SSD would be much faster.
Plus those disks will have enough to do having to keep the raid 6
array in sync... :-)

From what pretty much all of you have suggested, there shouldn't be
any changes in size with future disks I might purchase
but it's still best to leave a few MB just in case...

I will therefore leave 10MB at the end of each disk, pray for the best
and, in case it goes horribly wrong,
I'll blame all of you! Just kidding! LOL

Cheers,
Andrea

On Mon, May 27, 2013 at 10:11 AM, Adam Goryachev
<mailinglists@websitemanagers.com.au> wrote:
> On Mon, May 27, 2013 18:38, Andrea Bolandrina wrote:
>> Hi Tudor.
>>
>> All disks I have report the same size, even though they are different
>> brand/model
>
> Discussion on this list in the past would suggest the manufacturers have
> agreed to make equal sized drives in future, so this shouldn't really be
> an issue for 2TB drives or larger.
>
>> If the 2 WD RED bahave as they should I might buy some more as soon as
>> the others fail,
>> otherwise I will try something different (that's why I want to keep
>> the system as flexible as possible).
>
> Why not just put a 2G swap partition at the end of each drive, if needed,
> you can reduce the swap partition on the new drive if it is a bit smaller
> than the rest, you have up to 2G of flexibility.... (or 1G etc..) This
> means the space isn't wasted overall, but there is no issue if you need to
> make it a bit smaller/larger/etc...
>
>> The 2 samsung SMART infos are already "screaming for help" and,
>> actually, a third Samsung already failed last year.
>> Terrible drives, terrible support from Samsung that replaced the drive
>> (still under warranty) with a refurbished one that failed within 10
>> days from arrival.
>
> Personally, apart from a "batch" of WD Caviar Black drives that all failed
> within a week, of had really good experience with them, and also the WD
> RE4 drives.... I haven't yet tried the red, but would be interested to
> hear how you go.
>
> Regards,
> Adam
>