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Date:   Wed, 23 Dec 2020 02:25:19 +0100
From:   Matteo Croce <>
To:     Andreas Dilger <>
Cc:     Ext4 <>, Wang Shilong <>,
        "Theodore Y. Ts'o" <>
Subject: Re: discard and data=writeback

On Tue, Dec 22, 2020 at 11:53 PM Andreas Dilger <> wrote:
> On Dec 22, 2020, at 9:34 AM, Theodore Y. Ts'o <tytso@....EDU> wrote:
> >
> > On Tue, Dec 22, 2020 at 03:59:29PM +0100, Matteo Croce wrote:
> >>
> >> I'm issuing sync + sleep(10) after the extraction, so the writes
> >> should all be flushed.
> >> Also, I repeated the test three times, with very similar results:
> >
> > So that means the problem is not due to page cache writeback
> > interfering with the discards.  So it's most likely that the problem
> > is due to how the blocks are allocated and laid out when using
> > data=ordered vs data=writeback.
> >
> > Some experiments to try next.  After extracting the files with
> > data=ordered and data=writeback on a freshly formatted file system,
> > use "e2freefrag" to see how the free space is fragmented.  This will
> > tell us how the file system is doing from a holistic perspective, in
> > terms of blocks allocated to the extracted files.  (E2freefrag is
> > showing you the blocks *not* allocated, of course, but that's a mirror
> > image dual of the blocks that *are* allocated, especially if you start
> > from an identical known state; hence the use of a freshly formatted
> > file system.)
> >
> > Next, we can see how individual files look like with respect to
> > fragmentation.  This can be done via using filefrag on all of the
> > files, e.g:
> >
> >       find . -type f -print0  | xargs -0 filefrag
> >
> > Another way to get similar (although not identical) information is via
> > running "e2fsck -E fragcheck" on a file system.  How they differ is
> > especially more of a big deal on ext3 file systems without extents and
> > flex_bg, since filefrag tries to take into account metadata blocks
> > such as indirect blocks and extent tree blocks, and e2fsck -E
> > fragcheck does not; but it's good enough for getting a good gestalt
> > for the files' overall fragmentation --- and note that as long as the
> > average fragment size is at least a megabyte or two, some
> > fragmentation really isn't that much of a problem from a real-world
> > performance perspective.  People can get way too invested in trying to
> > get to perfection with 100% fragmentation-free files.  The problem
> > with doing this at the expense of all else is that you can end up
> > making the overall free space fragmentation worse as the file system
> > ages, at which point the file system performance really dives through
> > the floor as the file system approaches 100%, or even 80-90% full,
> > especially on HDD's.  For SSD's fragmentation doesn't matter quite so
> > much, unless the average fragment size is *really* small, and when you
> > are discarded freed blocks.
> >
> > Even if the files are showing no substantial difference in
> > fragmentation, and the free space is equally A-OK with respect to
> > fragmentation, the other possibility is the *layout* of the blocks are
> > such that the order in which they are deleted using rm -rf ends up
> > being less friendly from a discard perspective.  This can happen if
> > the directory hierarchy is big enough, and/or the journal size is
> > small enough, that the rm -rf requires multiple journal transactions
> > to complete.  That's because with mount -o discard, we do the discards
> > after each transaction commit, and it might be that even though the
> > used blocks are perfectly contiguous, because of the order in which
> > the files end up getting deleted, we end up needing to discard them in
> > smaller chunks.
> >
> > For example, one could imagine a case where you have a million 4k
> > files, and they are allocated contiguously, but if you get
> > super-unlucky, such that in the first transaction you delete all of
> > the odd-numbered files, and in second transaction you delete all of
> > the even-numbered files, you might need to do a million 4k discards
> > --- but if all of the deletes could fit into a single transaction, you
> > would only need to do a single million block discard operation.
> >
> > Finally, you may want to consider whether or not mount -o discard
> > really makes sense or not.  For most SSD's, especially high-end SSD's,
> > it probably doesn't make that much difference.  That's because when
> > you overwrite a sector, the SSD knows (or should know; this might not
> > be some really cheap, crappy low-end flash devices; but on those
> > devices, discard might not be making uch of a difference anyway), that
> > the old contents of the sector is no longer needed.  Hence an
> > overwrite effectively is an "implied discard".  So long as there is a
> > sufficient number of free erase blocks, the SSD might be able to keep
> > up doing the GC for those "implied discards", and so accelerating the
> > process by sending explicit discards after every journal transaction
> > might not be necessary.  Or, maybe it's sufficient to run "fstrim"
> > every week at Sunday 3am local time; or maybe even fstrim once a night
> > or fstrim once a month --- your mileage may vary.
> >
> > It's going to vary from SSD to SSD and from workload to workload, but
> > you might find that mount -o discard isn't buying you all that much
> > --- if you run a random write workload, and you don't notice any
> > performance degradation, and you don't notice an increase in the SSD's
> > write amplification numbers (if they are provided by your SSD), then
> > you might very well find that it's not worth it to use mount -o
> > discard.
> >
> > I personally don't bother using mount -o discard, and instead
> > periodically run fstrim, on my personal machines.  Part of that is
> > because I'm mostly just reading and replying to emails, building
> > kernels and editing text files, and that is not nearly as stressful on
> > the FTL as a full-blown random write workload (for example, if you
> > were running a database supporting a transaction processing workload).
> The problem (IMHO) with "-o discard" is that if it is only trimming
> *blocks* that were deleted, these may be too small to effectively be
> processed by the underlying device (e.g. the "super-unlucky" example
> above where interleaved 4KB file deletes result in 1M separate 4KB
> trim requests to the device, even when the *space* that is freed by
> the unlinks could be handled with far fewer large trim requests.
> There was a discussion previously ("introduce EXT4_BG_WAS_TRIMMED ...")
> about leveraging the persistent EXT4_BG_WAS_TRIMMED flag in the group
> descriptors, and having "-o discard" only track trim on a per-group
> basis rather than its current mode of doing trim on a per-block basis,
> and then use the same code internally as fstrim to do a trim of free
> blocks in that block group.
> Using EXT4_BG_WAS_TRIMMED and tracking *groups* to be trimmed would be
> a bit more lazy than the current "-o discard" implementation, but would
> be more memory efficient, and also more efficient for the device (fewer,
> larger trim requests submitted).  It would only need to track groups
> that have at least a reasonable amount of free space to be trimmed.  If
> the group doesn't have enough free blocks to trim now, it will be checked
> again in the future when more blocks are freed.


I gave it a quick run refreshing it for 5.10, but it doesn't seem to help.
Are there actions needed other than the patch itself?

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