Hi! > Note that even without MD raid, the file system issues IO's in file > system block size (4096 bytes normally) and most commodity storage > devices use a 512 byte sector size which means that we have to update 8 > 512b sectors. > > Drives can (and do) have multiple platters and surfaces and it is > perfectly normal to have contiguous logical ranges of sectors map to > non-contiguous sectors physically. Imagine a 4KB write stripe that > straddles two adjacent tracks on one platter (requiring a seek) or mapped > across two surfaces (requiring a head switch). Also, a remapped sector > can require more or less a full surface seek from where ever you are to > the remapped sector area of the drive. Yes, but ext3 was designed to handle the partial write (according to tytso). > These are all examples that can after a power loss, even a local > (non-MD) device, do a partial update of that 4KB write range of > sectors. Yes, but ext3 journal protects metadata integrity in that case. > In other words, this is not just an MD issue, it is entirely possible > even with non-MD devices. > > Also, when you enable the write cache (MD or not) you are buffering > multiple MB's of data that can go away on power loss. Far greater (10x) > the exposure that the partial RAID rewrite case worries about. Yes, that's what barriers are for. Except that they are not there on MD0/MD5/MD6. They actually work on local sata drives... Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
