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

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Date:   Thu, 31 Dec 2020 10:32:36 +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: Re: discard and data=writeback

Hi,

In the trace files, the amount of discard is almost the same in both modes.
(ordered: 1096MB / writeback: 1078MB) However, there is a big difference in
the average discard size per request. (ordered: 15.6KB / writeback: 34.2MB)
In ext4, when data is deleted, discard is immediately issued in writeback
mode. Therefore, the average size of discard commands is small and the
number of discard commands is large.
However, if it is not in the writeback mode, discard commands are issued by
JBD after merging them. Therefore, the average size of discard is large and
the number of discard commands is small.

In conclusion, since discard commands are not merged in the writeback mode,
many fragmented discard commands occur, so it affects the elapsed time of
many file deletion. And it is not abnormal behavior of ext4 file system.

Thanks,
Daejun