> I have a question that is related to this topic: Why > is there only a (tunable) 5 second threshold and not > also an additional threshold for the buffer size > (e.g. 50MB)? > > Sometimes I see my system writing huge amounts of > data to a zfs, but the disks staying idle for 5 > seconds, although the memory consumption is already > quite big and it really would make sense (from my > uneducated point of view as an observer) to start > writing all the data to disks. I think this leads to > the pumping effect that has been previously mentioned > in one of the forums here. > > Can anybody comment on this? > > TIA, > Thomas
because ZFS always writes to a new location on the disk, premature writing can often result in redundant work ... a single host write to a ZFS object results in the need to rewrite all of the changed data and meta-data leading to that object if a subsequent follow-up write to the same object occurs quickly, this entire path, once again, has to be recreated, even though only a small portion of it is actually different from the previous version if both versions were written to disk, the result would be to physically write potentially large amounts of nearly duplicate information over and over again, resulting in logically vacant bandwidth consolidating these writes in host cache eliminates some redundant disk writing, resulting in more productive bandwidth ... providing some ability to tune the consolidation time window and/or the accumulated cache size may seem like a reasonable thing to do, but I think that it's typically a moving target, and depending on an adaptive, built-in algorithm to dynamically set these marks (as ZFS claims it does) seems like a better choice ...Bill This message posted from opensolaris.org _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
