Re: Best way (only?) to setup SSD's for using TRIM

[Ric Wheeler <rwheeler@redhat.com>]

On 10/31/2012 10:11 AM, David Brown wrote:
> On 31/10/2012 14:12, Alexander Haase wrote:
>> Has anyone considered handling TRIM via an idle IO queue? You'd have to
>> purge queue items that conflicted with incoming writes, but it does get
>> around the performance complaint. If the idle period never comes, old
>> TRIMs can be silently dropped to lessen queue bloat.
>>
>
> I am sure it has been considered - but is it worth the effort and the 
> complications?  TRIM has been implemented in several filesystems (ext4 and, I 
> believe, btrfs) - but is disabled by default because it typically slows down 
> the system.  You are certainly correct that putting TRIM at the back of the 
> queue will avoid the delays it causes - but it still will not give any 
> significant benefit (except for old SSDs with limited garbage collection and 
> small over-provisioning ), and you have a lot of extra complexity to ensure 
> that a TRIM is never pushed back until after a new write to the same logical 
> sectors.

I think that you are vastly understating the need for discard support or what 
your first hand experience is, so let me  inject some facts into this thread 
from working on this for several years (with vendors) :)

Overview:

* In Linux, we have "discard" support which vectors down into the device 
appropriate method (TRIM for S-ATA, UNMAP/WRITE_SAME+UNMAP for SCSI, just 
discard for various SW only block devices)
* There is support for inline discard in many file systems (ext4, xfs, btrfs, 
gfs2, ...)
* There is support for "batched" discard (still online) via tools like fstrim

Every SSD device benefits from TRIM and the SSD companies test this code with 
the upstream community.

In our testing with various devices, the inline (mount -o discard) can have a 
performance impact so typically using the batched method is better.

For SCSI arrays (less an issue here on this list), the discard allows for 
over-provisioning of LUN's.

Device mapper has support (newly added) for dm-thinp targets which can do the 
same without hardware support.

>
> It would be much easier and safer, and give much better effect, to make sure 
> the block allocation procedure for filesystems emphasised re-writing old 
> blocks as soon as possible (when on an SSD).  Then there is no need for TRIM 
> at all.  This would have the added benefit of working well for compressed (or 
> sparse) hard disk image files used by virtual machines - such image files only 
> take up real disk space for blocks that are written, so re-writes would save 
> real-world disk space.

Above you are mixing the need for TRIM (which allows devices like SSD's to do 
wear levelling and performance tuning on physical blocks) with the virtual block 
layout of SSD devices. Please keep in mind that the block space advertised out 
to a file system is contiguous, but SSD's internally remapped the physical 
blocks aggressively. Think of physical DRAM and your virtual memory layout.

Doing a naive always allocate and reuse the lowest block would have horrendous 
performance impact on certain devices. Even on SSD's where seek is negligible, 
having to do lots of small IO's instead of larger, contiguous IO's is much slower.

Regards,

Ric


>
>> As far as parity consistency, bitmaps could track which stripes( and
>> blocks within those stripes) are expected to be out of parity( also
>> useful for lazy device init ). Maybe a bit-per-stripe map at the logical
>> device level and a bit-per-LBA bitmap at the stripe level?
>
> Tracking "no-sync" areas of a raid array is already high on the md raid 
> things-to-do list (perhaps it is already implemented - I lose track of which 
> features are planned and which are implemented). And yes, such no-sync 
> tracking would be useful here.  But it is complicated, especially for raid5/6 
> (raid1 is not too bad) - should TRIMs that cover part of a stripe be dropped?  
> Should the md layer remember them and coalesce them when it can TRIM a whole 
> stripe?  Should it try to track partial synchronisation within a stripe?
>
> Or should the md developers simply say that since supporting TRIM is not going 
> to have any measurable benefits (certainly not with the sort of SSD's people 
> use in raid arrays), and since TRIM slows down some operations, it is better 
> to keep things simple and ignore TRIM entirely?  Even if there are occasional 
> benefits to having TRIM, is it worth it in the face of added complication in 
> the code and the risk of errors?
>
> There /have/ been developers working on TRIM support on raid5.  It seems to 
> have been a complicated process.  But some people like a challenge!
>
>>
>> On the other hand, does it hurt if empty blocks are out of parity( due
>> to TRIM or lazy device init)? The parity recovery of garbage is still
>> garbage, which is what any sane FS expects from unused blocks. If and
>> when you do a parity scrub, you will spend a lot of time recovering
>> garbage and undo any good TRIM might have done, but usual drive
>> operation should quickly balance that out in a write-intensive
>> environment where idle TRIM might help.
>>
>
> Yes, it "hurts" if empty blocks are out of sync.  On obvious issue is that you 
> will get errors when scrubbing - the md layer has no way of knowing that these 
> are unimportant (assuming there is no no-sync tracking), so any real problems 
> will be hidden by the unimportant ones.
>
> Another issue is for RMW cycles on raid5.  Small writes are done by reading 
> the old data, reading the old parity, writing the new data and the new parity 
> - but that only works if the parity was correct across the whole stripe.  Even 
> if raid5 TRIM is restricted to whole stripes, a later small write to that 
> stripe will be a disaster if it is not in sync.
>
>
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