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

[Greg Freemyer]

On Thu, Aug 20, 2009 at 10:42 AM, Ric Wheeler<rwheeler@xxxxxxxxxx> wrote:
> On 08/20/2009 10:38 AM, 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.
>
> If the storage can return different data in a sequence of READ requests of
> the same sector (with no writes), there is nothing RAID could do. It would
> see total garbage...
>
>> 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)
>
> This should be safe if the MD bitmaps would prevent us from trying to
> READ/regenerate parity for that stripe...
>
> ric

The harder thing for mdraid is putting a stripe back in service.  If
even a single sector is written to a "discarded" stripe, the entire
stripe has to be written with determinate data that has the right
parity.

ie. Only full stripes can be discarded and only full-stripes can be
put back in service.

Greg
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