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4d10046613
[BUG] With added debugging, it turns out the following write sequence would cause extra read which is unnecessary: # xfs_io -f -s -c "pwrite -b 32k 0 32k" -c "pwrite -b 32k 32k 32k" \ -c "pwrite -b 32k 64k 32k" -c "pwrite -b 32k 96k 32k" \ $mnt/file The debug message looks like this (btrfs header skipped): partial rmw, full stripe=389152768 opf=0x0 devid=3 type=1 offset=32768 physical=323059712 len=32768 partial rmw, full stripe=389152768 opf=0x0 devid=1 type=2 offset=0 physical=67174400 len=65536 full stripe rmw, full stripe=389152768 opf=0x1 devid=3 type=1 offset=0 physical=323026944 len=32768 full stripe rmw, full stripe=389152768 opf=0x1 devid=2 type=-1 offset=0 physical=323026944 len=32768 partial rmw, full stripe=298844160 opf=0x0 devid=1 type=1 offset=32768 physical=22052864 len=32768 partial rmw, full stripe=298844160 opf=0x0 devid=2 type=2 offset=0 physical=277872640 len=65536 full stripe rmw, full stripe=298844160 opf=0x1 devid=1 type=1 offset=0 physical=22020096 len=32768 full stripe rmw, full stripe=298844160 opf=0x1 devid=3 type=-1 offset=0 physical=277872640 len=32768 partial rmw, full stripe=389152768 opf=0x0 devid=3 type=1 offset=0 physical=323026944 len=32768 partial rmw, full stripe=389152768 opf=0x0 devid=1 type=2 offset=0 physical=67174400 len=65536 ^^^^ Still partial read, even 389152768 is already cached by the first. write. full stripe rmw, full stripe=389152768 opf=0x1 devid=3 type=1 offset=32768 physical=323059712 len=32768 full stripe rmw, full stripe=389152768 opf=0x1 devid=2 type=-1 offset=32768 physical=323059712 len=32768 partial rmw, full stripe=298844160 opf=0x0 devid=1 type=1 offset=0 physical=22020096 len=32768 partial rmw, full stripe=298844160 opf=0x0 devid=2 type=2 offset=0 physical=277872640 len=65536 ^^^^ Still partial read for 298844160. full stripe rmw, full stripe=298844160 opf=0x1 devid=1 type=1 offset=32768 physical=22052864 len=32768 full stripe rmw, full stripe=298844160 opf=0x1 devid=3 type=-1 offset=32768 physical=277905408 len=32768 This means every 32K writes, even they are in the same full stripe, still trigger read for previously cached data. This would cause extra RAID56 IO, making the btrfs raid56 cache useless. [CAUSE] Commit |
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| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.