Hi!
I have an application which is using sqlite database in WAL mode. There is a
need for
periodic backups to a remote site without obstructing the normal operation.
Both read
and write access is required during the backup. At the moment I have system
which
first blocks checkpoints as otherwise backup was restarted too often. Backup is
done
to remote database implemented with sqlite vfs layer.
What happens in my tests is that all sqlite3_backup_step()'s complete without
actually
writing anything to remote database. Only during last step all pages are
written. This would
be annoying from progress monitoring point of view. However, as database mutex
is held
during this time it actually blocks all access to the database for a long
period of time.
Changing the backup step size does not help as all this happens at last step
regardless.
So, is this a bug? I'm I doing something wrong? What I was hoping was that
backup would
not hold database mutex while it is writing to the other database or at least
allow splitting the
work with sqlite3_backup_step() so that at least some work could be done while
backup is
in progress. I actually have strong impression that this worked better with
some older sqlite
version. Currently using 3.27.2.
Relevant part of the stack trace:
#8 0x00000000005dc870 in sqlite3OsWrite (id=0x7fc1a4120f98,
pBuf=0x7fc1a47b0e88, amt=<optimized out>, offset=<optimized out>)
at sqlite3.c:22229
#9 pager_write_pagelist (pPager=0x7fc1a41216f8, pList=0x7fc1a47c0ec0) at
sqlite3.c:54971
#10 0x00000000005bb1a5 in sqlite3PagerCommitPhaseOne (pPager=0x7fc1a41216f8,
zMaster=0x0, noSync=0)
at sqlite3.c:57050
#11 0x00000000005b968f in sqlite3_backup_step (p=0x7fc1a4056658,
nPage=<optimized out>) at sqlite3.c:74033
Seems that all database pages are written out in pager_write_pagelist() in
single loop.
Also, I'm open to other suggestions. I was contemplating to just copy the
database file
directly while WAL checkpoints are not done but I read some comments that
suggested
that it would not be a safe to do that.
Any help or suggestions would be appreciated!
Toni Tammisalo
ttamm...@iki.fi
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