63 sectors

[Avi Kivity <avi@qumranet.com>]

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?

-- 
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