Re: [PATCH, RFC] xfs: batched discard support

[Ric Wheeler <rwheeler@redhat.com>]

On 08/20/2009 11:43 AM, Rolf Eike Beer wrote:
> Mark Lord wrote:
>    
>> Ric Wheeler wrote:
>>      
>>> Note that returning consistent data is critical for devices that are
>>> used in a RAID group since you will need each RAID block that is used to
>>> compute the parity to continue to return the same data until you
>>> overwrite it with new data :-)
>>>
>>> If we have a device that does not support this (or is misconfigured not
>>> to do this), we should not use those devices in an MD group&  do discard
>>> against it...
>>>        
>> ..
>>
>> Well, that's a bit drastic.  But the RAID software should at least
>> not issue TRIM commands in ignorance of such.
>>
>> Would it still be okay to do the TRIMs when the entire parity stripe
>> (across all members) is being discarded?  (As opposed to just partial
>> data there being dropped)
>>      
> I think there might be a related usecase that could benefit from
> TRIM/UNMAP/whatever support in file systems even if the physical devices do
> not support that. I have a RAID5 at work with LVM over it. This week I deleted
> an old logical volume of some 200GB that has been moved to a different volume
> group, tomorrow I will start to replace all the disks in the raid with bigger
> ones. So if the LVM told the raid "hey, this space is totally garbage from now
> on" the raid would not have to do any calculation when it has to rebuild that
> but could simply write fixed patterns to all disks (e.g. 0 to first data, 0 to
> second data and 0 as "0 xor 0" to parity). With the knowledge that some of the
> underlying devices would support "write all to zero" this operation could be
> speed up even more, with "write all fixed pattern" every unused chunk would go
> down to a single write operation (per disk) on rebuild regardless which parity
> algorithm is used.
>    

In the SCSI world, RAID array vendors use "WRITE_SAME" to do this. For 
the SCSI discard, the write same command has a discard bit set if I 
remember correctly so you basically get what you are describing above.

ric

> And even if things are in use the RAID can benefit from such things. If we
> just define that every unmapped space will always be 0 when read and I write
> to a raid volume and the other part of the checksum calculation is unmapped
> checksumming becomes easy as we already know half of the values before: 0. So
> we can save the reads from the second data stripe and most of the calculation.
> "dd if=/dev/md0" on an unmapped space is more or less the same as "dd
> if=/dev/zero" than.
>
> I only fear that these things are too obviously as I would be the first to
> have this idea ;)
>
> Greetings,
>
> Eike
>