linux-ext4 - RE: Re: discard and data=writeback

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Date:   Wed, 30 Dec 2020 14:21:39 +0900
From:   Daejun Park <daejun7.park@...sung.com>
To:     Matteo Croce <mcroce@...ux.microsoft.com>,
        Daejun Park <daejun7.park@...sung.com>
CC:     "tytso@....edu" <tytso@....edu>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: RE: Re: discard and data=writeback

> 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.
> 
The full trace you shared on this thread seems contains only on writeback
mode. In the trace, discards are issued by each deletion file by rm.

If you share the full trace on ordered mode, it will help we analyze the
results. It is expected that number of discards will lower than writeback
mode, because discards can be merged on ordered mode.

Thanks,
Daejun