Messages in this thread | | | Date | Sat, 19 Mar 2022 14:31:49 -0400 | Subject | Re: [PATCH] btrfs: raid56: do blk_check_plugged check twice while writing | From | Sweet Tea Dorminy <> |
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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
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