> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] On Behalf Of Avi Kivity
> Sent: Wednesday, September 03, 2008 4:12 PM
> To: KVM list
> Cc: H. Peter Anvin
> Subject: 63 sectors
>
> Qemu sets the sectors-per-track setting of virtual disks to 63. This
> seems to be in accordance with the specs; drivers/ide/ide-disk.c says:
>
> > /*
> > * The ATA spec tells large drives to return
> > * C/H/S = 16383/16/63 independent of their size.
> > * Some drives can be jumpered to use 15 heads
> instead of 16.
> > * Some drives can be jumpered to use 4092 cyls
> instead of 16383.
> > */
> > if ((id->cyls == 16383
> > || (id->cyls == 4092 && id->cur_cyls == 16383)) &&
> > id->sectors == 63 &&
> > (id->heads == 15 || id->heads == 16) &&
> > (id->lba_capacity >= 16383*63*id->heads))
> > return 1;
>
> That setting has some unfortunate side effects. Partitioning
> tools will
> locate the first partition at the second cylinder, which is
> at the 63rd
> sector. This means that if the guest uses a 4K block
> filesystem on the
> first partition (an incredibly common occurance), then every single
> access will not be 4K aligned with respect to the virtual
> block device.
> This will cause fragmentation and read/modify/write cycles with:
>
> - qcow2 (which uses aligned 4K blocks)
> - any host filesystem which uses 4K blocks
> - any host disk which uses 4K blocks (not yet common)
>
> I can think of a few workarounds, all bad:
> - add a partitioning tool (or option to qemu-img) to format the disk,
> placing the first partition on the fourth cylinder, aligning
> it. tell
> the users not to wipe the disks out but instead install to one of the
> existing parititions
> - add a tool to optimize an existing disk by extending it and
> moving the
> partitions around so they are aligned. may break boot loaders.
> - make qcow4 use 512 byte sectors. will increase overhead
> and doesn't
> solve problems on the host filesystem and disk.
> - have qcow51 detect misaligned accesses and adjust itself somehow.
> doesn't help raw and other formats. likely very difficult.
>
> Does anybody know if scsi will have the same problems? Can anyone
> suggest other workarounds?
>
How about adding 1 sector offset when access disk image.
So that it looks like the 63rd sector, but it's indeed the 64th.
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