Commits on Sep 13, 2024

1. Adding Direct IO Support …

   Adding O_DIRECT support to ZFS to bypass the ARC for writes/reads.
   
   O_DIRECT support in ZFS will always ensure there is coherency between
   buffered and O_DIRECT IO requests. This ensures that all IO requests,
   whether buffered or direct, will see the same file contents at all
   times. Just as in other FS's , O_DIRECT does not imply O_SYNC. While
   data is written directly to VDEV disks, metadata will not be synced
   until the associated  TXG is synced.
   For both O_DIRECT read and write request the offset and requeset sizes,
   at a minimum, must be PAGE_SIZE aligned. In the event they are not,
   then EINVAL is returned unless the direct property is set to always (see
   below).
   
   For O_DIRECT writes:
   The request also must be block aligned (recordsize) or the write
   request will take the normal (buffered) write path. In the event that
   request is block aligned and a cached copy of the buffer in the ARC,
   then it will be discarded from the ARC forcing all further reads to
   retrieve the data from disk.
   
   For O_DIRECT reads:
   The only alignment restrictions are PAGE_SIZE alignment. In the event
   that the requested data is in buffered (in the ARC) it will just be
   copied from the ARC into the user buffer.
   
   For both O_DIRECT writes and reads the O_DIRECT flag will be ignored in
   the event that file contents are mmap'ed. In this case, all requests
   that are at least PAGE_SIZE aligned will just fall back to the buffered
   paths. If the request however is not PAGE_SIZE aligned, EINVAL will
   be returned as always regardless if the file's contents are mmap'ed.
   
   Since O_DIRECT writes go through the normal ZIO pipeline, the
   following operations are supported just as with normal buffered writes:
   Checksum
   Compression
   Encryption
   Erasure Coding
   There is one caveat for the data integrity of O_DIRECT writes that is
   distinct for each of the OS's supported by ZFS.
   FreeBSD - FreeBSD is able to place user pages under write protection so
             any data in the user buffers and written directly down to the
   	  VDEV disks is guaranteed to not change. There is no concern
   	  with data integrity and O_DIRECT writes.
   Linux - Linux is not able to place anonymous user pages under write
           protection. Because of this, if the user decides to manipulate
   	the page contents while the write operation is occurring, data
   	integrity can not be guaranteed. However, there is a module
   	parameter `zfs_vdev_direct_write_verify` that contols the
   	if a O_DIRECT writes that can occur to a top-level VDEV before
   	a checksum verify is run before the contents of the I/O buffer
           are committed to disk. In the event of a checksum verification
   	failure the write will return EIO. The number of O_DIRECT write
   	checksum verification errors can be observed by doing
   	`zpool status -d`, which will list all verification errors that
   	have occurred on a top-level VDEV. Along with `zpool status`, a
   	ZED event will be issues as `dio_verify` when a checksum
   	verification error occurs.
   
   ZVOLs and dedup is not currently supported with Direct I/O.
   
   A new dataset property `direct` has been added with the following 3
   allowable values:
   disabled - Accepts O_DIRECT flag, but silently ignores it and treats
   	   the request as a buffered IO request.
   standard - Follows the alignment restrictions  outlined above for
   	   write/read IO requests when the O_DIRECT flag is used.
   always   - Treats every write/read IO request as though it passed
              O_DIRECT and will do O_DIRECT if the alignment restrictions
   	   are met otherwise will redirect through the ARC. This
   	   property will not allow a request to fail.
   
   There is also a module paramter zfs_dio_enabled that can be used to
   force all reads and writes through the ARC. By setting this module
   paramter to 0, it mimics as if the  direct dataset property is set to
   disabled.
   
   Signed-off-by: Brian Atkinson <batkinson@lanl.gov>
   Co-authored-by: Mark Maybee <mark.maybee@delphix.com>
   Co-authored-by: Matt Macy <mmacy@FreeBSD.org>
   Co-authored-by: Brian Behlendorf <behlendorf@llnl.gov>

   

   4 people committed Sep 13, 2024
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