On Tue, Dec 29, 2009 at 2:21 PM, Jamie Lokier <jamie@xxxxxxxxxxxxx> wrote: > Avi Kivity wrote: >> Guests use this number as a hint for alignment and I/O request sizes. > > It's not just a hint. It is also the "radius of corruption on failed > write" - important for journalling filesystems and databases. > >> Given >> that modern disks have 4K block sizes, > > Do they, yet? Yes, there are WD disks in the wild with 4k blocks, although in this first transition phase the firmware hides the fact and emulates the old 512b sector. >> We probably need to make this configurable depending on machine type. It >> should be the default for -M 0.13 only as it can affect guest code paths. > > What about that Windows/Linux 4k sectors incompatibility thing, where > disks with 4k sectors have to sense whether the first partition starts > at 512-byte sector 63 (Linux) or 512-byte sector 1024 (or something; > Windows), and then adjust their 512-byte sector to 4k-sector mapping > so that 4k blocks within the partition are aligned to 4k sectors? Linux tools put the first partition at sector 63 (512-byte) to retain compatibility with Windows; Linux itself does not have any problem with different layouts. See e.g. [1] The problem seems to be limited to Win 5.x (XP, 2k3) and WD has an utility[2] to re-align partitions in this case, so I guess that they do cope fine with a 4k-aligned partition table, they just create it unaligned by default. > It has been discussed for hardware disk design with 4k sectors, and > somehow there were plans to map sectors so that the Linux partition > scheme results in nicely aligned filesystem blocks Ugh, I hope you're wrong ;-) AFAICS remapping will lead only to headaches... Linux does not have any problem with aligned partitions. Luca [1] http://thunk.org/tytso/blog/2009/02/20/aligning-filesystems-to-an-ssds-erase-block-size/ [2] http://support.wdc.com/product/download.asp?groupid=805&sid=123&lang=en -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
