Re: [PATCH 2/2] block: create ioctl to discard-or-zeroout a range of blocks

Andreas Dilger
2016-03-17T20:49:17Z

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On Mar 17, 2016, at 12:35 PM, Chris Mason <clm@fb.com> wrote:
>
> On Thu, Mar 17, 2016 at 10:47:29AM -0700, Linus Torvalds wrote:
>> On Wed, Mar 16, 2016 at 10:18 PM, Gregory Farnum <greg@gregs42.com> wrote:
>>>
>>> So we've not asked for NO_HIDE_STALE on the mailing lists, but I think
>>> it was one of the problems Sage had using xfs in his BlueStore
>>> implementation and was a big part of why it moved to pure userspace.
>>> FileStore might use NO_HIDE_STALE in some places but it would be
>>> pretty limited. When it came up at Linux FAST we were discussing how
>>> it and similar things had been problems for us in the past and it
>>> would've been nice if they were upstream.
>>
>> Hmm.
>>
>> So to me it really sounds like somebody should cook up a patch, but we
>> shouldn't put it in the upstream kernel until we get numbers and
>> actual "yes, we'd use this" from outside of google.
>
> We haven't had internal tiers yelling at us for fallocate performance,
> so I'm unlikely to suggest it, just because its a potential
> privacy leak we'd have to educate people about. What I'd be more likely
> to use is code inside the filesystem like this:
>
> somefs_fallocate() {
> if (trim_can_really_zero(my_device)) {
> trim
> allocate a regular extent
> return
> } else {
> do normal fallocate
> }
> }

We were discussing almost this very same thing in the ext4 concall today.

Ted initially didn't think it was worthwhile to implement, but after looking
at the whitelist for SATA SSDs it seems that there are enough devices on the
market that support the ATA_HORKAGE_ZERO_AFTER_TRIM to make this approach
worthwhile to implement.

Also, if the ext4 extent size was limited it might even be possible to do
this efficiently enough with write_same on HDD devices.

> Then the out of tree patch (for google or whoever) becomes a hack to
> flip trim_can_really_zero on a given block device. The rest of us can
> use explicit interfaces from the hardware when deciding what we want
> preallocation to mean.

This might be a bit trickier, since this would affect all zero/trim
operations, not just ones for uninitialized data extents.

> It gets messy for crcs in btrfs, so we'd need the old fashioned
> preallocation anyway. But the database workloads where this matters
> aren't our target right now, so its more an ext4/xfs thing anyway.

Cheers, Andreas

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