On 19/01/23 18:14, Song Liu wrote:
On Wed, Jan 23, 2019 at 8:53 AM Michal Soltys <soltys@xxxxxxxx> wrote:
Hi,
From kernel documentation:
"In write-back mode, MD reports IO completion to upper layer (usually
filesystems) right after the data hits cache disk."
Does "data hits cache disk" mean the actual flush of the journal device as well ?
Or is it staged for later moment when the full-stripe write(s) to the actual raid
device are to be performed ?
MD layer will write the data to the cache disk, but not flush it. Flush happens
when upper layer (file system or application) issues flush (fsync, etc.).
Does this answer the question?
Yes, thanks.
For the record, my question came after checking 2016 presentation (
https://events.static.linuxfound.org/sites/events/files/slides/2016_vault_write_journal_cache_v2.pdf
). The slide titled "RAID - 456 Write Cache: Write Path" kind of
suggested that the flush might be happening, which was a bit suspicious
(or I misunderstood that slide).
Thanks,
Song
