path: root/man
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authorStefan Behrens <>2012-05-16 18:51:28 +0200
committerDavid Sterba <>2013-03-19 18:23:58 +0100
commit4739e7332cd70c18cdd783f37246d3cdcabff98b (patch)
treee6c16f16b038ecd454d65259759431b9ab60c207 /man
parentc535e2f7a7bb9d0ca1fe815383d3ff0f48cca182 (diff)
Btrfs-progs: make scrub IO priority configurable
The btrfs tool is changed in order to support command line parameters to configure the IO priority of the scrub tasks. Also the default is changed. The default IO priority for scrub is the idle class now. The behavior is the same as when one would type 'ionice ... btrfs scrub start ...' or 'ionice ... btrfs scrub resume ...' (without this patch applied). The only reason for adding this to the btrfs tool is that it was not documented and not obvious that it worked like this, that all internal scrub tasks inherited the IO priority values of the btrfs tool that is starting or resuming the scrub operation. Note that after applying the patch it is no longer possible to set the IO priority using ionice since the btrfs tool always configures the priority in order to run in the idle class by default. Some basic performance measurements have been done with the goal to measure which IO priority for scrub gives the best overall disk data throughput. The kernel was configured to use the CFQ IO scheduler with default configuration and without support for throttling. The summary is, that the more the disk head movements are avoided, the faster the overall disk transfer capacity is, which is not really a big surprise. Therefore it makes sense that the best data throughput was measured setting the scrub IO priority and the scrub readahead IO priority to the idle class priority. Running with idle class IO priority means that scrub and scrub readahead IO is paused while other tasks access the disk. Doing the tasks one after the other instead of concurrently avoids many disk head movements. The overall data throughput of rotating disks is improved this way. However, if it is desired to have the scrub task done within a reasonable time, and if at the same time the filesystem is heavily loaded, the idle IO priority should be avoided. Otherwise the scrub operation will never take place and thus never terminate. The best effort IO priority class with the subclass 7 (the lowest one in the best effort class) is recommended in the case of always heavily loaded hard disks. If the filesystem is not loaded all the time and leaves some idle slots for scrub, the idle class IO priority is recommended. The idle class now is the default if the scrub operation is started with the btrfs-progs tools. Note that the patch that sets the scrub readahead IO priority to the idle class is a seperate patch, this needs to be done in the kernel. Signed-off-by: Stefan Behrens <>
Diffstat (limited to 'man')
1 files changed, 18 insertions, 4 deletions
diff --git a/man/ b/man/
index 94f4ffea..08e5fd79 100644
--- a/man/
+++ b/man/
@@ -47,11 +47,11 @@ btrfs \- control a btrfs filesystem
\fBbtrfs\fP \fBreplace cancel\fP \fI<path>\fP
-\fBbtrfs\fP \fBscrub start\fP [-Bdqru] {\fI<path>\fP|\fI<device>\fP}
+\fBbtrfs\fP \fBscrub start\fP [-Bdqru] [-c ioprio_class -n ioprio_classdata] {\fI<path>\fP|\fI<device>\fP}
\fBbtrfs\fP \fBscrub cancel\fP {\fI<path>\fP|\fI<device>\fP}
-\fBbtrfs\fP \fBscrub resume\fP [-Bdqru] {\fI<path>\fP|\fI<device>\fP}
+\fBbtrfs\fP \fBscrub resume\fP [-Bdqru] [-c ioprio_class -n ioprio_classdata] {\fI<path>\fP|\fI<device>\fP}
\fBbtrfs\fP \fBscrub status\fP [-d] {\fI<path>\fP|\fI<device>\fP}
@@ -355,11 +355,16 @@ Cancel a running device replace operation.
\fBscrub start\fP [-Bdqru] {\fI<path>\fP|\fI<device>\fP}
+\fBscrub start\fP [-Bdqru] [-c ioprio_class -n ioprio_classdata] {\fI<path>\fP|\fI<device>\fP}
Start a scrub on all devices of the filesystem identified by \fI<path>\fR or on
a single \fI<device>\fR. Without options, scrub is started as a background
process. Progress can be obtained with the \fBscrub status\fR command. Scrubbing
involves reading all data from all disks and verifying checksums. Errors are
corrected along the way if possible.
+The default IO priority of scrub is the idle class. The priority can be configured similar to the
+.BR ionice (1)
@@ -373,6 +378,14 @@ Quiet. Omit error messages and statistics.
Read only mode. Do not attempt to correct anything.
.IP -u 5
Scrub unused space as well. (NOT IMPLEMENTED)
+.IP -c 5
+Set IO priority class (see
+.BR ionice (1)
+.IP -n 5
+Set IO priority classdata (see
+.BR ionice (1)
@@ -384,7 +397,7 @@ If a \fI<device>\fR is given, the corresponding filesystem is found and
\fBscrub cancel\fP behaves as if it was called on that filesystem.
-\fBscrub resume\fP [-Bdqru] {\fI<path>\fP|\fI<device>\fP}
+\fBscrub resume\fP [-Bdqru] [-c ioprio_class -n ioprio_classdata] {\fI<path>\fP|\fI<device>\fP}
Resume a canceled or interrupted scrub cycle on the filesystem identified by
\fI<path>\fR or on a given \fI<device>\fR. Does not start a new scrub if the
last scrub finished successfully.
@@ -446,4 +459,5 @@ and not suitable for any uses other than benchmarking and review.
Please refer to the btrfs wiki for
further details.
-.BR mkfs.btrfs (8)
+.BR mkfs.btrfs (8),
+.BR ionice (1)