Re: [PATCH] btrfs: raid56: do blk_check_plugged check twice while writing
From: Sweet Tea Dorminy
Date: Sat Mar 19 2022 - 14:31:57 EST
Hi Yusuf:
On 3/18/22 21:18, Yusuf Khan wrote:
Do the check to see if the drive was connected twice in case that
the first was a fluke.
Block plugging is not actually about checking the connectivity of the
drive; it's about batching IO submission. Specifically, when a client of
the block layer (a filesystem or DM device, for instance) knows that it
needs to send a whole batch of IO to disk from a thread, it can request
the thread's be temporarily delayed, with blk_start_plug()
(block/blk-core.c has a lengthy great comment about it). Then it submits
its batch of requests. Finally, it requests the plug be released. This
is useful because it can allow better IO by releasing a large,
potentially better ordered batch at once.
In this particular case, to my understanding, blk_check_plug() checks
whether the current thread is plugged, and, if it is, it adds the rbio
to a list; when the plug expires, perhaps from the thread going to
sleep, the plug callback gets called, specifically btrfs_raid_unplug(),
which sorts the pending IOs.
As such, it shouldn't be necessary to check whether the thread is
plugged twice. Your change description is a little light on *why* you'd
like to check twice; maybe I'm missing something.
Hope this helps,
Sweet Tea