On 02/22/16 18:56, Damien Le Moal wrote:
2) Write back of dirty pages to SMR block devices:
Dirty pages of a block device inode are currently processed using the
generic_writepages function, which can be executed simultaneously
by multiple contexts (e.g sync, fsync, msync, sync_file_range, etc).
Mutual exclusion of the dirty page processing being achieved only at
the page level (page lock & page writeback flag), multiple processes
executing a "sync" of overlapping block ranges over the same zone of
an SMR disk can cause an out-of-LBA-order sequence of write requests
being sent to the underlying device. On a host managed SMR disk, where
sequential write to disk zones is mandatory, this result in errors and
the impossibility for an application using raw sequential disk write
accesses to be guaranteed successful completion of its write or fsync
requests.
Using the zone information attached to the SMR block device queue
(introduced by Hannes), calls to the generic_writepages function can
be made mutually exclusive on a per zone basis by locking the zones.
This guarantees sequential request generation for each zone and avoid
write errors without any modification to the generic code implementing
generic_writepages.
This is but one possible solution for supporting SMR host-managed
devices without any major rewrite of page cache management and
write-back processing. The opinion of the audience regarding this
solution and discussing other potential solutions would be greatly
appreciated.
Hello Damien,
Is it sufficient to support filesystems like BTRFS on top of SMR drives
or would you also like to see that filesystems like ext4 can use SMR
drives ? In the latter case: the behavior of SMR drives differs so
significantly from that of other block devices that I'm not sure that we
should try to support these directly from infrastructure like the page
cache. If we look e.g. at NAND SSDs then we see that the characteristics
of NAND do not match what filesystems expect (e.g. large erase blocks).
That is why every SSD vendor provides an FTL (Flash Translation Layer),
either inside the SSD or as a separate software driver. An FTL
implements a so-called LFS (log-structured filesystem). With what I know
about SMR this technology looks also suitable for implementation of a
LFS. Has it already been considered to implement an LFS driver for SMR
drives ? That would make it possible for any filesystem to access an SMR
drive as any other block device. I'm not sure of this but maybe it will
be possible to share some infrastructure with the LightNVM driver
(directory drivers/lightnvm in the Linux kernel tree). This driver
namely implements an FTL.
Bart.
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