Re: Software RAID and TRIM

[David Brown <david@westcontrol.com>]

On 19/07/2011 11:29, Lutz Vieweg wrote:
> On 07/18/2011 10:18 PM, David Brown wrote:
>> You don't need to fill an erase block for writing - writes are done
>> as write blocks (I think 4K is the norm).
>
> You are right on that. Those sectors in a partially used erase block
> that have not been written to since the last erase of the whole erase
> block can be written to as good as sectors in completely empty erase
> blocks.
>
>
>> My main point about TRIM being expensive is the effect it has on
>> the block IO queue, regardless of the implementation in the SSD.
>
> Because of those effects on the block-IO-queue, the user-space
> work-around we implemented to discard the SSDs our RAID-1s consist of
> will not discard "one area on all SSDs at a time", but rather iterate
> first through all unused areas on one SSD, then iterate through the
> same list of areas on the second SSD.
>

Do you take the arrays off-line during this process, or at least make
them read-only?  If not, how do you ensure that the lists are valid?

> The effect of this is very much to our liking: While we can see
> near-100%-utilization on one SSD at a time during the discards, the
> other SSD will happily service the readers, and even the writes that
> go to the /dev/md* device are buffered in main memory long enough
> that we do not really see a significantly bad impact on the service.
> (This might be different, though, if the discards were done during
> peak-write-load times of the day.)
>
>
>> I really hope your SSD's return zeros for TRIM'ed blocks
>
> For RAID-1, the only consequence of not doing so is just that
> "data-check" runs may result in a > 0 mismatch_cnt. It does not
> destroy any of your data, and as long as I have two SSDs in a RAID,
> both of which give a non-error result when reading a sector, I would
> have no indication of "which of the returned sector contents to
> prefer", anyway.
>
> (I admit that for health monitoring it is useful to have a meaningful
>  mismatch_cnt.)
>
>> and that you are sure all your TRIMs are in full raid stripes -
>> otherwise you will /seriously/ mess up your raid arrays.
>
> Again, for RAID0/1 (even 10) I don't see why this would harm any
> data.
>

Fair enough for RAID1.  Just don't try it with RAID5!