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Re: [PATCH 0/7] qcow2: compressed write cache

From: Max Reitz
Subject: Re: [PATCH 0/7] qcow2: compressed write cache
Date: Wed, 10 Feb 2021 11:00:05 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0

On 09.02.21 19:51, Vladimir Sementsov-Ogievskiy wrote:
09.02.2021 21:41, Denis V. Lunev wrote:
On 2/9/21 9:36 PM, Vladimir Sementsov-Ogievskiy wrote:
09.02.2021 19:39, Vladimir Sementsov-Ogievskiy wrote:
09.02.2021 17:47, Max Reitz wrote:
On 09.02.21 15:10, Vladimir Sementsov-Ogievskiy wrote:
09.02.2021 16:25, Max Reitz wrote:
On 29.01.21 17:50, Vladimir Sementsov-Ogievskiy wrote:
Hi all!

I know, I have several series waiting for a resend, but I had to
to another task spawned from our customer's bug.

Original problem: we use O_DIRECT for all vm images in our
product, it's
the policy. The only exclusion is backup target qcow2 image for
compressed backup, because compressed backup is extremely slow with
O_DIRECT (due to unaligned writes). Customer complains that backup
produces a lot of pagecache.

So we can either implement some internal cache or use fadvise
Backup has several async workes, which writes simultaneously, so
in both
ways we have to track host cluster filling (before dropping the
corresponding to the cluster).  So, if we have to track anyway,
try to implement the cache.

I wanted to be excited here, because that sounds like it would be
very easy to implement caching.  Like, just keep the cluster at
free_byte_offset cached until the cluster it points to changes,
then flush the cluster.

The problem is that chunks are written asynchronously.. That's why
this all is not so easy.

But then I see like 900 new lines of code, and I’m much less

Idea is simple: cache small unaligned write and flush the cluster

Performance result is very good (results in a table is time of
compressed backup of 1000M disk filled with ones in seconds):

“Filled with ones” really is an edge case, though.

Yes, I think, all clusters are compressed to rather small chunks :)

---------------  -----------  -----------
                   backup(old)  backup(new)
ssd:hdd(direct)  3e+02        4.4
ssd:hdd(cached)  5.7          5.4
---------------  -----------  -----------

So, we have benefit even for cached mode! And the fastest thing is
O_DIRECT with new implemented cache. So, I suggest to enable the new
cache by default (which is done by the series).

First, I’m not sure how O_DIRECT really is relevant, because I
don’t really see the point for writing compressed images.

compressed backup is a point

(Perhaps irrelevant, but just to be clear:) I meant the point of
using O_DIRECT, which one can decide to not use for backup targets
(as you have done already).

Second, I find it a bit cheating if you say there is a huge
improvement for the no-cache case, when actually, well, you just
added a cache.  So the no-cache case just became faster because
there is a cache now.

Still, performance comparison is relevant to show that O_DIRECT as
is unusable for compressed backup.

(Again, perhaps irrelevant, but:) Yes, but my first point was
exactly whether O_DIRECT is even relevant for writing compressed

Well, I suppose I could follow that if O_DIRECT doesn’t make much
sense for compressed images, qemu’s format drivers are free to
introduce some caching (because technically the cache.direct
option only applies to the protocol driver) for collecting
compressed writes.

Yes I thought in this way, enabling the cache by default.

That conclusion makes both of my complaints kind of moot.


Third, what is the real-world impact on the page cache?  You
described that that’s the reason why you need the cache in qemu,
because otherwise the page cache is polluted too much.  How much
is the difference really?  (I don’t know how good the compression
ratio is for real-world images.)

Hm. I don't know the ratio.. Customer reported that most of RAM is
polluted by Qemu's cache, and we use O_DIRECT for everything except
for target of compressed backup.. Still the pollution may relate to
several backups and of course it is simple enough to drop the cache
after each backup. But I think that even one backup of 16T disk may
pollute RAM enough.

Oh, sorry, I just realized I had a brain fart there.  I was
referring to whether this series improves the page cache pollution.
But obviously it will if it allows you to re-enable O_DIRECT.

Related to that, I remember a long time ago we had some discussion
about letting qemu-img convert set a special cache mode for the
target image that would make Linux drop everything before the last
offset written (i.e., I suppose fadvise() with
POSIX_FADV_SEQUENTIAL).  You discard that idea based on the fact
that implementing a cache in qemu would be simple, but it isn’t,
really.  What would the impact of POSIX_FADV_SEQUENTIAL be?  (One
advantage of using that would be that we could reuse it for
non-compressed images that are written by backup or qemu-img

The problem is that writes are async. And therefore, not sequential.

In theory, yes, but all compressed writes still goes through
qcow2_alloc_bytes() right before submitting the write, so I wonder
whether in practice the writes aren’t usually sufficiently
sequential to make POSIX_FADV_SEQUENTIAL work fine.

Yes, allocation is sequential. But writes are not.. Reasonable, I
should at least bench it. So we should set POSIX_FADV_SEQUENTIAL for
the whole backup target before the backup? Will try. Still, I expect
that my cache will show better performance anyway. Actually,
comparing cached (by pagecache) vs my cache we have 5.7 -> 4.4, i.e.
20% faster, which is significant (still, yes, would be good to check
it on more real case than all-ones).

I have to track the writes and wait until the whole cluster is
filled. It's simple use fadvise as an option to my cache: instead
of caching data and write when cluster is filled we can instead

(I don’t remember why that qemu-img discussion died back then.)

Fourth, regarding the code, would it be simpler if it were a pure
write cache?  I.e., on read, everything is flushed, so we don’t
have to deal with that.  I don’t think there are many valid cases
where a compressed image is both written to and read from at the
same time. (Just asking, because I’d really want this code to be
simpler.  I can imagine that reading from the cache is the least
bit of complexity, but perhaps...)

Hm. I really didn't want to support reads, and do it only to make
it possible to enable the cache by default.. Still read function is
really simple, and I don't think that dropping it will simplify the
code significantly.

That’s too bad.

So the only question I have left is what POSIX_FADV_SEQUENTIAL
actually would do in practice.

will check.

Checked that if I mark the whole file by FADV_SEQUENTIAL, cache is not

[root@kvm fadvise]# cat a.c
#define _GNU_SOURCE
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <getopt.h>
#include <string.h>
#include <stdbool.h>

int main(int argc, char *argv[])
     int fd;
     int i;
     char mb[1024 * 1024];
     int open_flags = O_RDWR | O_CREAT | O_EXCL;
     int fadv_flags = 0;
     int fadv_final_flags = 0;
     int len = 1024 * 1024;
     bool do_fsync = false;

     for (i = 1; i < argc - 1; i++) {
         const char *arg = argv[i];

         if (!strcmp(arg, "direct")) {
             open_flags |= O_DIRECT;
         } else if (!strcmp(arg, "seq")) {
             fadv_flags = POSIX_FADV_SEQUENTIAL;
         } else if (!strcmp(arg, "dontneed")) {
             fadv_flags = POSIX_FADV_DONTNEED;
         } else if (!strcmp(arg, "final-dontneed")) {
             fadv_final_flags = POSIX_FADV_DONTNEED;
         } else if (!strcmp(arg, "fsync")) {
             do_fsync = true;
         } else {
             fprintf(stderr, "unknown: %s\n", arg);
             return 1;

     fd = open(argv[argc - 1], open_flags);

     if (fd < 0) {
         fprintf(stderr, "failed to open\n");
         return 1;

     if (fadv_flags) {
         posix_fadvise(fd, 0, 100 * 1024 * 1024, fadv_flags);
     for (i = 0; i < 100; i++) {
         write(fd, mb, len);
     if (fadv_final_flags) {
         posix_fadvise(fd, 0, 100 * 1024 * 1024, fadv_final_flags);
     if (do_fsync) {


[root@kvm fadvise]# gcc a.c
[root@kvm fadvise]# rm -f x; ./a.out seq x; fincore x
  100M 25600  100M x
[root@kvm fadvise]# rm -f x; ./a.out dontneed x; fincore x
  100M 25600  100M x
[root@kvm fadvise]# rm -f x; ./a.out final-dontneed x; fincore x
   36M  9216  100M x
[root@kvm fadvise]# rm -f x; ./a.out seq fsync x; fincore x
  100M 25600  100M x
[root@kvm fadvise]# rm -f x; ./a.out dontneed fsync x; fincore x
  100M 25600  100M x
[root@kvm fadvise]# rm -f x; ./a.out final-dontneed fsync x; fincore x
   36M  9216  100M x
[root@kvm fadvise]# rm -f x; ./a.out direct x; fincore x
  0B     0   0B x

Backup-generated pagecache is a formal trash, it will be never used.
And it's bad that it can displace another good pagecache. So the best
thing for backup is direct mode + proposed cache.

What a shame.

Thanks a lot for testing.

I think that the original intention of Max is about POSIX_FADV_DONTNEED
One should call this fadvise for just fully written cluster.

I had hoped that SEQUENTIAL would just work, magically.

This should work, but:

 - as we see from test above, POSIX_FADV_DONTNEED don't remove the whole cache (see final-dontneed)  - as I said we'll have to track writes, so the cache will be the same, just instead of postponed-write operation we'll do fadvise.

Hmm. Still, in this way, we will not need some difficult things in my proposed cache.

So, it may be reasonable to at least split the big patch so that

  - first part brings writes / full-cluster tracking and fadvice

 - second part adds caching-wrties option, corresponding flush code and additional performance

Does it make sense?

I think the fadvise solution would have been nice if it gave us something magical that we could also use for normal qemu-img convert (or backup) operations, but as that doesn’t seem to be the case, I don’t think it makes too much sense to implement something like that. (I imagine doing fadvise also creates the need to implement new block to call into file-posix and so on.)

I’d propose I take some time to look at your patch as-is and then I report back.


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