linux-ext4 - Re: discard and data=writeback

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Date:   Tue, 29 Dec 2020 14:42:22 +0100
From:   Matteo Croce <mcroce@...ux.microsoft.com>
To:     daejun7.park@...sung.com
Cc:     "tytso@....edu" <tytso@....edu>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: discard and data=writeback

On Tue, Dec 29, 2020 at 6:41 AM Daejun Park <daejun7.park@...sung.com> wrote:
>
> Hi,
>
> > # dmesg |grep EXT4-fs |tail -1
> > [ 1594.829833] EXT4-fs (nvme0n1p1): mounted filesystem with ordered
> > data mode. Opts: data=ordered,discard
> > # blktrace /dev/nvme0n1 & sleep 1 ; time rm -rf /media/linux-5.10/ ; kill $!
> > [1] 3032
> >
> > real    0m1.328s
> > user    0m0.063s
> > sys     0m1.231s
> > # === nvme0n1 ===
> >   CPU  0:                    0 events,        0 KiB data
> >   CPU  1:                    0 events,        0 KiB data
> >   CPU  2:                    0 events,        0 KiB data
> >   CPU  3:                 1461 events,       69 KiB data
> >   CPU  4:                    1 events,        1 KiB data
> >   CPU  5:                    0 events,        0 KiB data
> >   CPU  6:                    0 events,        0 KiB data
> >   CPU  7:                    0 events,        0 KiB data
> >   Total:                  1462 events (dropped 0),       69 KiB data
> >
> >
> > # dmesg |grep EXT4-fs |tail -1
> > [ 1734.837651] EXT4-fs (nvme0n1p1): mounted filesystem with writeback
> > data mode. Opts: data=writeback,discard
> > # blktrace /dev/nvme0n1 & sleep 1 ; time rm -rf /media/linux-5.10/ ; kill $!
> > [1] 3069
> >
> > real    1m30.273s
> > user    0m0.139s
> > sys     0m3.084s
> > # === nvme0n1 ===
> >   CPU  0:               133830 events,     6274 KiB data
> >   CPU  1:                21878 events,     1026 KiB data
> >   CPU  2:                46365 events,     2174 KiB data
> >   CPU  3:                98116 events,     4600 KiB data
> >   CPU  4:               290902 events,    13637 KiB data
> >   CPU  5:                10926 events,      513 KiB data
> >   CPU  6:                76861 events,     3603 KiB data
> >   CPU  7:                17855 events,      837 KiB data
> >   Total:                696733 events (dropped 0),    32660 KiB data
> >
>
> In this result, there is few IO in ordered mode.
>
> As I understand (please correct this if I am wrong), with writeback +
> discard, ext4_issue_discard is called immediately at each rm command.
> However, with ordered mode, ext4_issue_discard is called when end of
> committing a transaction to pace with the corresponding transaction.
> It means, they are not discarded yet.
>
> Even with ordered mode, if sync is called after rm command,
> ext4_issue_discard can be called due to transaction commit.
> So, I think you will get similar results form writeback mode with sync
> command.
>

Hi,

that's what I get with data=ordered if I issue a sync after the removal:

# time rm -rf /media/linux-5.10/ ; sync ; kill $!

real    0m1.569s
user    0m0.044s
sys     0m1.508s
#
 === nvme0n1 ===
 CPU  0:                10980 events,      515 KiB data
 CPU  1:                    0 events,        0 KiB data
 CPU  2:                    0 events,        0 KiB data
 CPU  3:                   26 events,        2 KiB data
 CPU  4:                 3601 events,      169 KiB data
 CPU  5:                    0 events,        0 KiB data
 CPU  6:                21786 events,     1022 KiB data
 CPU  7:                    0 events,        0 KiB data
 Total:                 36393 events (dropped 0),     1706 KiB data

Still way less transactions than writeback.

Cheers,
-- 
per aspera ad upstream