On Fri, Apr 2, 2010 at 16:24, Edward Ned Harvey <solar...@nedharvey.com> wrote: >> The purpose of the ZIL is to act like a fast "log" for synchronous >> writes. It allows the system to quickly confirm a synchronous write >> request with the minimum amount of work. > > Bob and Casper and some others clearly know a lot here. But I'm hearing > conflicting information, and don't know what to believe. Does anyone here > work on ZFS as an actual ZFS developer for Sun/Oracle? Can claim "I can > answer this question, I wrote that code, or at least have read it?" > > Questions to answer would be: > > Is a ZIL log device used only by sync() and fsync() system calls? Is it > ever used to accelerate async writes?
sync() will tell the filesystems to flush writes to disk. sync() will not use ZIL, it will just start a new TXG, and could return before the writes are done. fsync() is what you are interested in. > > Suppose there is an application which sometimes does sync writes, and > sometimes async writes. In fact, to make it easier, suppose two processes > open two files, one of which always writes asynchronously, and one of which > always writes synchronously. Suppose the ZIL is disabled. Is it possible > for writes to be committed to disk out-of-order? Meaning, can a large block > async write be put into a TXG and committed to disk before a small sync > write to a different file is committed to disk, even though the small sync > write was issued by the application before the large async write? Remember, > the point is: ZIL is disabled. Question is whether the async could > possibly be committed to disk before the sync. > Writers from a TXG will not be used until the whole TXG is committed to disk. Everything from a half written TXG will be ignored after a crash. This means that the order of writes within a TXG is not important. The only way to do a sync write without ZIL is to start a new TXG after the write. That costs a lot so we have the ZIL for sync writes. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
