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Date: Wed, 30 Dec 2020 16:16:42 +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: Re: discard and data=writeback
On Wed, Dec 30, 2020 at 6:21 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.
> >
> 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.
>
Hi,
I did the same blktrace with data=ordered,discard
Find it here:
https://drive.google.com/file/d/1gqffP9WPCME3_81xlXAQCiDlTK-Gqv4_/view?usp=sharing
Thanks,
--
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