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Date: Sun, 22 May 2011 07:26:29 -0400 From: Christoph Hellwig <hch@...radead.org> To: Alex Bligh <alex@...x.org.uk> Cc: linux-kernel@...r.kernel.org Subject: Re: REQ_FLUSH, REQ_FUA and open/close of block devices > So, the file in question is not mmap'd (it's an nbd disk). fsync() / > fdatasync() is too expensive as it will sync everything. As far as I can > tell, this is no more dangerous re metadata than fdatasync() which also > does not sync metadata. I had read the last sentence as "this system > call does not *necessarily* flush disk write caches" (meaning "if you > haven't mounted e.g. ext3 with barriers=1, then you can't ensure write > caches write through"), as opposed to "will not ever flush disk write > caches", and given mounting ext3 without barriers=1 produces no FUA or > FLUSH commands in normal operation anyway (as far as light debugging > can see) that's not much of a loss. ext3 without barriers does not gurantee any data integrity and will lose your data in an eye blink if you have a large enough cache. fdatasync is equivalent to fsync except that it does not flush non-essential metadata (basically just timestamps in practice), but it does flush metadata requried to find the data again, e.g. allocation information and extent maps. sync_file_range does nothing but flush out pagecache content - it means you basically won't get your data back in case of a crash if you either: a) have a volatile write cache in your disk (e.g. any normal SATA disk) b) are using a sparse file on a filesystem c) are using a fallocate-preallocated file on a filesystem d) use any file on a COW filesystem like btrfs e.g. it only does anything useful for you if you do not have a volatile write cache, and either use a raw block device node, or just overwrite an already fully allocated (and not preallocated) file on a non-COW filesystem. > But rather than trying to justify myself: what is the best way to > emulate FUA, i.e. ensure a specific portion of a file is synced before > returning, without ensuring the whole lot is synced (which is far too > slow)? The only other option I can see is to open the file with a second > fd, mmap the chunk of the file (it may be larger than the available > virtual address space), mysnc it with MS_SYNC, then fsync, then munmap > and close, and hope the fsync doesn't spit anything else out. This > seems a little excessive, and I don't even know whether it would work. You can have a second FD with O_DSYNC open and write to that. But for NBD and Linux guest that won't make any different yet. While REQ_FUA is a separate flag so far it's only used in combination with REQ_FLUSH, so the only pattern you'll see REQ_FUA used in is: REQ_FLUSH REQ_FUA which means there's no data but the one just written in the cache. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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