Re: 3.10: discard/trim support on md-raid1?

[David Lang]

Part of this depends on the exact failure mode. I've seen cases where drives 
fail, and the drive does a bunch of retries, then the OS does a bunch of 
retries, and eventually the read fails, but in the meantime, everything stalls 
for a long time.

I've even seen the same thing in at least one case where there was a hardware 
RAID card in use.

David Lang

On Tue, 16 Jul 2013, Justin Piszcz wrote:

> Thanks for the replies,
>
> After some further testing..
> When I ran a repair on the md's sync_action, the system would reduce
> I/O to the RAID-1 to 14kb/s or even less when it hit a certain number
> of blocks and effectively locked the system every time.
> It turned out to be a bad SSD (it also failed Intel's Secure Erase), I RMA'd it.
> Interesting though that it did not drop out of the array but froze the
> system (the failure scenario was odd).
>
> Justin.
>
>
>
> On Tue, Jul 16, 2013 at 3:15 AM, NeilBrown <neilb@xxxxxxx> wrote:
> > On Sat, 13 Jul 2013 06:34:19 -0400 "Justin Piszcz" <jpiszcz@xxxxxxxxxxxxxxx>
> > wrote:
> >
> > > Hello,
> > >
> > > Running 3.10 and I see the following for an md-raid1 of two SSDs:
> > >
> > > Checking /sys/block/md1/queue:
> > > add_random: 0
> > > discard_granularity: 512
> > > discard_max_bytes: 2147450880
> > > discard_zeroes_data: 0
> > > hw_sector_size: 512
> > > iostats: 0
> > > logical_block_size: 512
> > > max_hw_sectors_kb: 32767
> > > max_integrity_segments: 0
> > > max_sectors_kb: 512
> > > max_segment_size: 65536
> > > max_segments: 168
> > > minimum_io_size: 512
> > > nomerges: 0
> > > nr_requests: 128
> > > optimal_io_size: 0
> > > physical_block_size: 512
> > > read_ahead_kb: 8192
> > > rotational: 1
> > > rq_affinity: 0
> > > scheduler: none
> > > write_same_max_bytes: 0
> > >
> > > What should be seen:
> > > rotational: 0
> >
> > What has "rotational" got to do with "supports discard"?
> > There may be some correlation, but it isn't causal.
> >
> > > And possibly:
> > > discard_zeroes_data: 1
> >
> > This should be set as the 'or' of the same value from component devices.  And
> > does not enable or disable the use of discard.
> >
> > I don't think that "does this device support discard" appears in sysfs.
> >
> > I believe trim does work on md/raid1 if the underlying devices all support it.
> >
> > NeilBrown
> >
> >
> >
> > > Can anyone confirm if there is a workaround to allow TRIM when using
> > > md-raid1?
> > >
> > > Some related discussion here:
> > > http://us.generation-nt.com/answer/md-rotational-attribute-help-206571222.ht
> > > ml
> > > http://www.progtown.com/topic343938-ssd-strange-itself-conducts.html
> > >
> > >
> > > Justin.
> > >
> > >
> > > --
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