Re: [PATCH v3 0/6] Composefs: an opportunistically sharing verified image filesystem

[Gao Xiang <hsiangkao@linux.alibaba.com>]

On 2023/2/2 15:17, Gao Xiang wrote:
> 
> 
> On 2023/2/2 14:37, Amir Goldstein wrote:
>> On Wed, Feb 1, 2023 at 1:22 PM Gao Xiang <hsiangkao@linux.alibaba.com> wrote:
>>>
>>>
>>>
>>> On 2023/2/1 18:01, Gao Xiang wrote:
>>>>
>>>>
>>>> On 2023/2/1 17:46, Alexander Larsson wrote:
>>>>
>>>> ...
>>>>
>>>>>>
>>>>>>                                     | uncached(ms)| cached(ms)
>>>>>> ----------------------------------|-------------|-----------
>>>>>> composefs (with digest)           | 326         | 135
>>>>>> erofs (w/o -T0)                   | 264         | 172
>>>>>> erofs (w/o -T0) + overlayfs       | 651         | 238
>>>>>> squashfs (compressed)                | 538         | 211
>>>>>> squashfs (compressed) + overlayfs | 968         | 302
>>>>>
>>>>>
>>>>> Clearly erofs with sparse files is the best fs now for the ro-fs +
>>>>> overlay case. But still, we can see that the additional cost of the
>>>>> overlayfs layer is not negligible.
>>>>>
>>>>> According to amir this could be helped by a special composefs-like mode
>>>>> in overlayfs, but its unclear what performance that would reach, and
>>>>> we're then talking net new development that further complicates the
>>>>> overlayfs codebase. Its not clear to me which alternative is easier to
>>>>> develop/maintain.
>>>>>
>>>>> Also, the difference between cached and uncached here is less than in
>>>>> my tests. Probably because my test image was larger. With the test
>>>>> image I use, the results are:
>>>>>
>>>>>                                     | uncached(ms)| cached(ms)
>>>>> ----------------------------------|-------------|-----------
>>>>> composefs (with digest)           | 681         | 390
>>>>> erofs (w/o -T0) + overlayfs       | 1788        | 532
>>>>> squashfs (compressed) + overlayfs | 2547        | 443
>>>>>
>>>>>
>>>>> I gotta say it is weird though that squashfs performed better than
>>>>> erofs in the cached case. May be worth looking into. The test data I'm
>>>>> using is available here:
>>>>
>>>> As another wild guess, cached performance is a just vfs-stuff.
>>>>
>>>> I think the performance difference may be due to ACL (since both
>>>> composefs and squashfs don't support ACL).  I already asked Jingbo
>>>> to get more "perf data" to analyze this but he's now busy in another
>>>> stuff.
>>>>
>>>> Again, my overall point is quite simple as always, currently
>>>> composefs is a read-only filesystem with massive symlink-like files.
>>>> It behaves as a subset of all generic read-only filesystems just
>>>> for this specific use cases.
>>>>
>>>> In facts there are many options to improve this (much like Amir
>>>> said before):
>>>>     1) improve overlayfs, and then it can be used with any local fs;
>>>>
>>>>     2) enhance erofs to support this (even without on-disk change);
>>>>
>>>>     3) introduce fs/composefs;
>>>>
>>>> In addition to option 1), option 2) has many benefits as well, since
>>>> your manifest files can save real regular files in addition to composefs
>>>> model.
>>>
>>> (add some words..)
>>>
>>> My first response at that time (on Slack) was "kindly request
>>> Giuseppe to ask in the fsdevel mailing list if this new overlay model
>>> and use cases is feasable", if so, I'm much happy to integrate in to
>>> EROFS (in a cooperative way) in several ways:
>>>
>>>    - just use EROFS symlink layout and open such file in a stacked way;
>>>
>>> or (now)
>>>
>>>    - just identify overlayfs "trusted.overlay.redirect" in EROFS itself
>>>      and open file so such image can be both used for EROFS only and
>>>      EROFS + overlayfs.
>>>
>>> If that happened, then I think the overlayfs "metacopy" option can
>>> also be shown by other fs community people later (since I'm not an
>>> overlay expert), but I'm not sure why they becomes impossible finally
>>> and even not mentioned at all.
>>>
>>> Or if you guys really don't want to use EROFS for whatever reasons
>>> (EROFS is completely open-source, used, contributed by many vendors),
>>> you could improve squashfs, ext4, or other exist local fses with this
>>> new use cases (since they don't need any on-disk change as well, for
>>> example, by using some xattr), I don't think it's really hard.
>>>
>>
>> Engineering-wise, merging composefs features into EROFS
>> would be the simplest option and FWIW, my personal preference.
>>
>> However, you need to be aware that this will bring into EROFS
>> vfs considerations, such as  s_stack_depth nesting (which AFAICS
>> is not see incremented composefs?). It's not the end of the world, but this
>> is no longer plain fs over block game. There's a whole new class of bugs
>> (that syzbot is very eager to explore) so you need to ask yourself whether
>> this is a direction you want to lead EROFS towards.
> 
> I'd like to make a seperated Kconfig for this.  I consider this just because
> currently composefs is much similar to EROFS but it doesn't have some ability
> to keep real regular file (even some README, VERSION or Changelog in these
> images) in its (composefs-called) manifest files. Even its on-disk super block
> doesn't have a UUID now [1] and some boot sector for booting or some potential
> hybird formats such as tar + EROFS, cpio + EROFS.
> 
> I'm not sure if those potential new on-disk features is unneeded even for
> future composefs.  But if composefs laterly supports such on-disk features,
> that makes composefs closer to EROFS even more.  I don't see disadvantage to
> make these actual on-disk compatible (like ext2 and ext4).
> 
> The only difference now is manifest file itself I/O interface -- bio vs file.
> but EROFS can be distributed to raw block devices as well, composefs can't.
> 
> Also, I'd like to seperate core-EROFS from advanced features (or people who
> are interested to work on this are always welcome) and composefs-like model,
> if people don't tend to use any EROFS advanced features, it could be disabled
> from compiling explicitly.

Apart from that, I still fail to get some thoughts (apart from unprivileged
mounts) how EROFS + overlayfs combination fails on automative real workloads
aside from "ls -lR" (readdir + stat).

And eventually we still need overlayfs for most use cases to do writable
stuffs, anyway, it needs some words to describe why such < 1s difference is
very very important to the real workload as you already mentioned before.

And with overlayfs lazy lookup, I think it can be close to ~100ms or better.

> 
>>
>> Giuseppe expressed his plans to make use of the composefs method
>> inside userns one day. It is not a hard dependency, but I believe that
>> keeping the "RO efficient verifiable image format" functionality (EROFS)
>> separate from "userns composition of verifiable images" (overlayfs)
>> may benefit the userns mount goal in the long term.
> 
> If that is needed, I'm very happy to get more detailed path of this from
> some discussion in LSF/MM/BPF 2023: how we get this (userns) reliably in
> practice.
> 
> As of code lines, core EROFS on-disk format is quite simple (I don't think
> total LOC is a barrier), if you see
>    fs/erofs/data.c
>    fs/erofs/namei.c
>    fs/erofs/dir.c
> 
> or
>     erofs_super_block
>     erofs_inode_compact
>     erofs_inode_extended
>     erofs_dirent
> 
> but for example, fs/erofs/super.c which is just used to enable EROFS advanced
> features is almost 1000LOC now.  But most code is quite trivial, I don't think
> these can cause any difference to userns plan.
> 
> Thanks,
> Gao Xiang
> 
> [1] https://lore.kernel.org/r/CAOQ4uxjm7i+uO4o4470ACctsft1m18EiUpxBfCeT-Wyqf1FAYg@mail.gmail.com/
> 
>>
>> Thanks,
>> Amir.