Re: hammer snapshot blocks on high file system activity

[Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>]

:Hi,
:
:I'm doing a "cpdup /usr/src /hammer/src" and a "hammer snapshot
:/hammer/snap-%Y" at the same time. "hammer snapshot" takes as long as
:"cpdup". Indeed, it does not return until the "cpdup" is completed.
:
:I think it's a problem in "hammer sync". But taking a snapshot is
:useless if one has to wait until no more file activity happens (which is
:rarely the case on a server).
:
:Regards,
:
:   Michael

    It isn't waiting for the activity to stop, it is trying to sync out
    the caches.  It should be able to do that in parallel with running
    activity but /usr/src is only ~450MB or so and it is likely that
    a large chunk of the target copy will have been cached, depending
    on how much memory you have.

    It is possible to query the last synchronized transaction id
    and generate a softlink based on that, without doing a new sync.
    This would give you a snapshot as-of 0-60 seconds ago verses 'now'.
    That is not usually what the user desires, though.

    You'd need to copy more data, on the order of a few gigs, to measure
    how long it takes snapshot to stage out the caches.  There could be
    a bug there but I'm pretty sure it is coded properly.

    Remember that hard drives cannot do 70MBytes/sec worth of random I/O,
    they can only do that rate when reading or writing large linear swaths.
    When you get into more random I/O the data rate will drop to
    5-15 MBytes/sec.

					-Matt
					Matthew Dillon 
					<dillon@backplane.com>