Re: xfs hardware RAID alignment over linear lvm

[Stan Hoeppner <stan@hardwarefreak.com>]

On 9/26/2013 4:58 PM, Dave Chinner wrote:
> On Thu, Sep 26, 2013 at 04:22:30AM -0500, Stan Hoeppner wrote:
>> On 9/26/2013 3:55 AM, Stewart Webb wrote:
>>> Thanks for all this info Stan and Dave,
>>>
>>>> "Stripe size" is a synonym of XFS sw, which is su * #disks.  This is the
>>>> amount of data written across the full RAID stripe (excluding parity).
>>>
>>> The reason I stated Stripe size is because in this instance, I have 3ware
>>> RAID controllers, which refer to
>>> this value as "Stripe" in their tw_cli software (god bless manufacturers
>>> renaming everything)
>>>
>>> I do, however, have a follow-on question:
>>> On other systems, I have similar hardware:
>>> 3x Raid Controllers
>>> 1 of them has 10 disks as RAID 6 that I would like to add to a logical
>>> volume
>>> 2 of them have 12 disks as a RAID 6 that I would like to add to the same
>>> logical volume
>>>
>>> All have the same "Stripe" or "Strip Size" of 512 KB
>>>
>>> So if I where going to make 3 seperate xfs volumes, I would do the
>>> following:
>>> mkfs.xfs -d su=512k sw=8 /dev/sda
>>> mkfs.xfs -d su=512k sw=10 /dev/sdb
>>> mkfs.xfs -d su=512k sw=10 /dev/sdc
>>>
>>> I assume, If I where going to bring them all into 1 logical volume, it
>>> would be best placed to have the sw value set
>>> to a value that is divisible by both 8 and 10 - in this case 2?
>>
>> No.  In this case you do NOT stripe align XFS to the storage, because
>> it's impossible--the RAID stripes are dissimilar.  In this case you use
>> the default 4KB write out, as if this is a single disk drive.
>>
>> As Dave stated, if you format a concatenated device with XFS and you
>> desire to align XFS, then all constituent arrays must have the same
>> geometry.
>>
>> Two things to be aware of here:
>>
>> 1.  With a decent hardware write caching RAID controller, having XFS
>> alined to the RAID geometry is a small optimization WRT overall write
>> performance, because the controller is going to be doing the optimizing
>> of final writeback to the drives.
>>
>> 2. Alignment does not affect read performance.
> 
> Ah, but it does...
> 
>> 3.  XFS only performs aligned writes during allocation.
> 
> Right, and it does so not only to improve write performance, but to
> also maximise sequential read performance of the data that is
> written, especially when multiple files are being read
> simultaneously and IO latency is important to keep low (e.g.
> realtime video ingest and playout).

Absolutely correct, as Dave always is.  As my workloads are mostly
random, as are those of others I consult in other fora, I sometimes
forget the [multi]streaming case.  Which is not good, as many folks
choose XFS specifically for [multi]streaming workloads.  My remarks to
this audience should always reflect that.  Apologies for my oversight on
this occasion.

>> What really makes a difference as to whether alignment will be of
>> benefit to you, and how often, is your workload.  So at this point, you
>> need to describe the primary workload(s) of your systems we're discussing.
> 
> Yup, my thoughts exactly...
> 
> Cheers,
> 
> Dave.
> 

-- 
Stan

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