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:
Quoted 4 lines
/*
* 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.Quoted 8 lines
* 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?
Signature · 1 line
error compiling committee.c: too many arguments to function
Quoted 15 lines · Avi Kivity
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.csays:Quoted 4 lines
/* * 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.Quoted 8 lines
* 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;
Quoted 1 line
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:
Oh, the tales I could tell of big disk hardware vendors that cache their storage platforms on 4k aligned blocks and have horrible performance for reads across a block boundary and only provide drivers for Windows to format LUNs with 4k alignment.
AFAIK, this is actually a relic limitation of the IBM PC-BIOS hard disk geometry functions and the MS-DOS partitioning scheme. The BIOS provides only five bits for sector number in a track and zero is invalid. And, sectors have to claim to be aligned on a cylinder boundary except for the first partition which is permitted to begin on the second head of the first cylinder (so sectors/track reserved sectors and the mis-alignment).
Quoted 4 lines
- qcow2 (which uses aligned 4K blocks)
- any host filesystem which uses 4K blocks
- any host disk which uses 4K blocks (not yet common)
Quoted 9 lines
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.
A solution used on another platform I'm familiar with is for the platform's storage drivers to report disk geometry as having have 32 sectors per track. The native partitioning tool will then always create partitions with 32 sector alignment. It's possible a BIOS change could be made to achieve that.
Quoted 1 line
Does anybody know if scsi will have the same problems? Can anyone suggest other workarounds?
If you are using a guest platform like linux and don't need multiple file systems on the same [virtual] disk then just format the whole disk rather than partition it:
# mkfs -t ext3 /dev/sdb
(instead of /dev/sdb1, 2, 3, etc)
Quoted 2 lines · David Mair
A solution used on another platform I'm familiar with is for the platform's storage drivers to report disk geometry as having have 32 sectors per track. The native partitioning tool will then always create partitions with 32 sector alignment. It's possible a BIOS change could be made to achieve that.
I tried 60 sectors per track, but Windows saw the disk as an 8GB disk and refused to format. Linux had similar problems.
Quoted 7 lines
Quoted 1 line
Does anybody know if scsi will have the same problems? Can anyone suggest other workarounds?
If you are using a guest platform like linux and don't need multiple file systems on the same [virtual] disk then just format the whole disk rather than partition it:
# mkfs -t ext3 /dev/sdb(instead of
/dev/sdb1, 2, 3, etc)
That won't work with the installers (at least not simply), and doesn't allow for a swap partition.
Signature · 1 line
error compiling committee.c: too many arguments to function
Quoted 5 lines
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
Well, that one slightly modified could work out quite well, at least for linux.
As far I know the linux kernel uses the geometry provided by the storage adapter only in case the disk is blank. If there is a partition table present on the disk, the linux kernel will calculate the geometry based on that.
Background for this is that scsi disks have no disk geometry, they just have a bunch of sectors. The scsi hostadapter bioses have to pull out some geometry out of thin air to satisfy ms-dos and real os boot loaders, and there was no standard on how to do that. Thus moving disks from one scsi adapter to another may result in hba-reported and on-disk geometry being different, and the only sane way to deal with that is using the on-disk geometry. With LBA this is much less an issue these days though.
So qemu-img could create a partition table, containing one partition, hinting 32 sectors/cylinder. Linux should keep that geometry then, even when distro installers are deleting the partition and creating their own scheme.
Dunno how what *BSD or Windows guests will deal with that though.
cheers,
Gerd
Quoted 4 lines · Gerd Hoffmann
So qemu-img could create a partition table, containing one partition, hinting 32 sectors/cylinder. Linux should keep that geometry then, even when distro installers are deleting the partition and creating their own scheme.
Scratch that, tested it, didn't work :-(
cheers,
Gerd
Quoted 17 lines · Avi Kivity
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.
Quoted 2 lines
Does anybody know if scsi will have the same problems? Can anyone suggest other workarounds?
This is not just limited to KVM. The right thing is for partitioning tools to realize that this isn't 1981 and actually align partitions to block boundaries. Windows Vista aligns partitions to megabyte boundaries; I personally think that is somewhat excessive, but aligning partitions to 128K (or possibly 64K) would be better.
This idiotic default damages virtualizers, flash drives, hardware RAID, and pretty soon spinning media as well.
-hpa
Quoted 57 lines
-----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 sectorsQemu sets the sectors-per-track setting of virtual disks to 63. This seems to be in accordance with the specs;
drivers/ide/ide-disk.csays:Quoted 4 lines
/*
* 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 headsinstead of 16.
Quoted 1 line
* Some drives can be jumpered to use 4092 cyls
instead of 16383.
Quoted 7 lines
*/ 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.
Quoted 3 lines
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.
Quoted 2 lines · Liu Yu-B13201 wrote:
How about adding 1 sector offset when access disk image. So that it looks like the 63rd sector, but it's indeed the 64th.
The problem with that is you make it suck on OSes which actually partition sanely.
-hpa
Quoted 7 lines
-----Original Message-----
From: H. Peter Anvin [mailto:[email protected]]
Sent: Thursday, September 04, 2008 11:33 AM
To: Liu Yu-B13201
Cc: Avi Kivity; KVM list
Subject: Re: 63 sectors
Quoted 7 lines
Quoted 2 lines · Liu Yu-B13201 wrote:
How about adding 1 sector offset when access disk image. So that it looks like the 63rd sector, but it's indeed the 64th.
The problem with that is you make it suck on OSes which actually partition sanely.
You mean guest OSes?
Guest OSes never know this offset, adding one sector offset is in qemu and transparent to them.
Quoted 8 lines · Liu Yu-B13201 wrote:
Quoted 5 lines
Quoted 2 lines · Liu Yu-B13201 wrote:
How about adding 1 sector offset when access disk image. So that it looks like the 63rd sector, but it's indeed the 64th.
The problem with that is you make it suck on OSes which actually partition sanely.
You mean guest OSes?
Guest OSes never know this offset, adding one sector offset is in qemu and transparent to them.
I mean guest OSes.
And no, they won't know the difference, but you are suggesting optimizing for guest OSes who are legacy/broken *AT THE EXPENSE* of guest OSes who do things sanely.
As partition and install utilities get a clue, there are going to be less of the former and more of the latter.
-hpa
This can be left to the userspace to pass as an option, to optimize for relevant guests, since the one running the userspace probably knows which OS he will run on it, so he can pass "-sector64" as an optimization.
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of H. Peter Anvin
Sent: Thursday, September 04, 2008 6:57 AM
To: Liu Yu-B13201
Cc: Avi Kivity; KVM list
Subject: Re: 63 sectors
Quoted 9 lines · Liu Yu-B13201 wrote:
Quoted 6 lines
Quoted 2 lines · Liu Yu-B13201 wrote:
How about adding 1 sector offset when access disk image. So that it looks like the 63rd sector, but it's indeed the 64th.
The problem with that is you make it suck on OSes which actually partition sanely.
You mean guest OSes?
Guest OSes never know this offset, adding one sector offset is in qemu and transparent to them.
I mean guest OSes.
And no, they won't know the difference, but you are suggesting optimizing for guest OSes who are legacy/broken *AT THE EXPENSE* of guest OSes who do things sanely.
As partition and install utilities get a clue, there are going to be less of the former and more of the latter.
-hpa
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Quoted 2 lines · Itamar Heim
This can be left to the userspace to pass as an option, to optimize for relevant guests, since the one running the userspace probably knows which OS he will run on it, so he can pass "-sector64" as an optimization.
Or rather -drive ...,offset=512
Signature · 1 line
error compiling committee.c: too many arguments to function
Quoted 3 lines · Avi Kivity
- 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
I thought that updating qemu-img to have quasi-fdisk functionality was a good idea myself. Is there a respected libfdisk.so thing that could be used to abstract most of the work ?
Quoted 9 lines · Ian Kirk
Quoted 3 lines · Avi Kivity
- 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
I thought that updating qemu-img to have quasi-fdisk functionality was a good idea myself. Is there a respected libfdisk.so thing that could be used to abstract most of the work ?
libparted ?
Laurent
Signature · 3 lines
----------------- [email protected] ------------------ "La perfection est atteinte non quand il ne reste rien à ajouter mais quand il ne reste rien à enlever." Saint Exupéry
Quoted 9 lines · Ian Kirk
Quoted 3 lines · Avi Kivity
- 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
I thought that updating qemu-img to have quasi-fdisk functionality was a good idea myself. Is there a respected libfdisk.so thing that could be used to abstract most of the work ?
libparted, although I wouldn't call it "respectable".
Writing a partition table to an otherwise empty disk is actually dirt simple.
-hpa
Quoted 3 lines · H. Peter Anvin
libparted, although I wouldn't call it "respectable".
That was meant as a sarcastic remark. Sorry for the confusion.
-hpa
Quoted 3 lines · Ian Kirk
I thought that updating qemu-img to have quasi-fdisk functionality was a good idea myself. Is there a respected libfdisk.so thing that could be used to abstract most of the work ?
Exporting textual output and piping that to sfdisk is one approach.