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

Eric Sandeen
2016-03-17T00:34:04Z

On 3/16/16 7:15 PM, Theodore Ts'o wrote:
> On Wed, Mar 16, 2016 at 03:45:49PM -0600, Andreas Dilger wrote:
>>> Clearly, the performance hit of unwritten extent conversion is large
>>> enough to tempt people to ask for no-hide-stale. But I'd rather hear
>>> that directly from a developer, Ceph or otherwise.
>>
>> I suspect that this gets significantly worse if you are running with
>> random writes instead of sequential overwrites. With sequential overwrites
>> there is only a single boundary between init and uninit extents, so at
>> most one extra extent in the tree. The above performance deltas will also
>> be much larger when real disks are involved and seek latency is a factor.
>
> It will vary a lot depending on your use case. If you are running
> with data=ordered, and with journalled enabled, then even if it is a
> single extent that is modified, the fact that a journal transaction
> involved, with a forced data block flush to avoid revealing stale
> data, that is certainly going to be measurable.
>
> The other thing is if you are worried about tail latency, which is a
> major concern at Google[1], and you are running your disks close to
> flat out, the fact that you have to do an extra seek to update the
> extent tree is a seek that you can't be using for useful work --- and
> worse, could delay a low-latency read from completing within your SLO.
>
> [1] https://research.google.com/pubs/pub44830.html
>
> Part of what's challenging with giving numbers is that it's trivially
> easy to give some worst case scneario where the numbers are really
> terrible. A random 4k random write benchmark into an fallocated file,
> eeven with XFS, would have pretty bad numbers, But of course people
> wouldn't say that it's very realistic. But those are the easiest to
> get.
>
> The most realistic numbers are going to be a lot harder to get, and
> wouldn't necessarily make a lot of sense without revealing a lot
> proprietary information. I will say that Google does have a fairly
> large number of disks[2] and so even a small fractional percentage
> gain multipled by gazillions of disks starts turning into a dollar
> number with enough zeros that people really sit up and take notice.
> I'll also note that map reduce can be quite nasty as far as random I/O
> is concerned[3], and while map reduce jobs are often not high priority
> jobs, they can interfere with low-latency reads from important
> applications (e.g., web search, user-visible gmail operations, etc.)
>
> [2] https://what-if.xkcd.com/63/
> [3] https://pdfs.semanticscholar.org/6238/e5f0fd807f634f5999701c7aa6a09d88dfc8.pdf
>
> So I'm not sure what numbers I can really give that would satisfy
> people. Doing a random write fio job is not hard, and will result in
> fairly impressive numbers. If that's enough, then either I can do
> this, or Chris Mason can reproduce his experiment using XFS (which
> would presumably eliminate the excuse that it's because ext4 sucks at
> extent operations). But if that's not going to convince people, then
> I'd much rather not waste my time.
>
> Besides, at Google it's easy enough for me to maintain the patch
> out-of-tree. It's the Ceph folks who would need to at the very least,
> have such a patch ship in Red Hat Enterprise Linux. So it's probably
> better for them to justify it, if numbers are really necessary.

I may have lost the thread at this point, with poor Darrick's original
patch submission devolving into a long thread about a NO_HIDE_STALE patch
used at Google, but I don't *think* Ceph ever asked for NO_HIDE_STALE.

At least I can't find any indication of that.

Am I missing something? cc'ing Greg on this one in case I am.

-Eric