On Fri, 13 Mar 2009, Marco Colombo wrote: > Hi, I'm a long time user of both PostgreSQL and LVM. So far I've been quite > happy with both. But a recent thread on the PostgreSQL list made me > unconfortable. What is this thing they're referring to, fsync()'s being > ignored? Makes me feel like I'm running on thin ice, without even > knowing. Before I start phasing out LVM from all my PostgreSQL installations > (as they suggest), I'd like to hear some kind of confirmation. > http://archives.postgresql.org/pgsql-general/2009-03/msg00204.php The discussion doesn't make a lot of sense. fsync() is a filesystem call - it can't possibly be handled (or ignored) at a lower level because the lowel level doesn't know which blocks belong to the file. I *can* imagine that perhaps the raw block writes used by the filesystem code might be ignored - or improperly cached. Clearly, they are not ignored (filesystems do get updated) - so if there is any substance to the charge, it must be that LVM reorders writes somehow. Caching doesn't really break anything - it is the *reordering* of writes that could be a problem. A "write barrier" says "finish these writes before you start any more, but otherwise reorder how you like". I did some experiments with iostat, and I am convinced that LVM does not itself do any reordering of writes. Here is my theory as to what is really going on: LVM is not really "ignoring" fsync(), because it would never see it. However, in the presence of hardware writeback caching in disk drives, fsync() would need to tell the hardware to "finish all these writes before you start any more" (i.e. - a write barrier) for fsync() to be effective. I suspect that LVM simply fails to pass the *write barrier* through to underlying layers (i.e. ignores the writebarrier call). Thus, you should be fine if you simply turn off writeback caching in your disk drives. If you could guarrantee that disk drives would be powered long enough after the main system stops - even a drive write back cache would not be much of a risk. -- Stuart D. Gathman <stuart@bmsi.com> Business Management Systems Inc. Phone: 703 591-0911 Fax: 703 591-6154 "Confutatis maledictis, flammis acribus addictis" - background song for a Microsoft sponsored "Where do you want to go from here?" commercial. _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
