[dm-crypt] status of dm-crypt (alignment issues) on a SSD

[dm-crypt] status of dm-crypt (alignment issues) on a SSD

[mike dentifrice]

Hello,

I'm about to install a laptop (using Debian Squeeze) that ships a SSD
instead of a HDD for the first time (using GPT partitionning, then
dm-crypt over a partition, then LVM2 to make a physical volume out of
the crypto device, and logical volumes underneath).

I've done some searching/reading on the alignment issues involving SSD,
and most threads end up stating that things will eventually work "out of
the box" at some point. Most of them are quite old though, so I'd like
to know what the status on this alignment issue is, and, if possible,
what software versions are required for things to work properly (I
understood that linux >= 2.6.33 was a good starting point, but what
about cryptsetup, lvm2, util-linux, etc.?).

Here are the software versions I'm likely to find within the latest
debian-installer snapshot:

  - linux-image-2.6.32-5-amd64 2.6.32-28
  - util-linux 2.17.2-3.3
  - dmsetup 2:1.02.48-4
  - lvm2 2.02.66-4
  - cryptsetup 2:1.1.3-4

Any hint would be greatly appreciated (including pointers to up-to-date
HOWTOs which I may have missed). Also, if you guys know which specific
feature of linux 2.6.33 is required, it would allow me to check whether
it's been backported to Debian's 2.6.32 or not.

Thanks a lot,

-- 
mike dentifrice <fluor@poivron.org>

Re: [dm-crypt] status of dm-crypt (alignment issues) on a SSD

[Luca Berra]

On Wed, Dec 01, 2010 at 03:51:24PM +0100, mike dentifrice wrote:
>Any hint would be greatly appreciated (including pointers to up-to-date
>HOWTOs which I may have missed). Also, if you guys know which specific
>feature of linux 2.6.33 is required, it would allow me to check whether
>it's been backported to Debian's 2.6.32 or not.
i dont know about debian and which software version has which feature,
but:

what you need on kernel side is having in sysfs
under /sys/block/<device>
queue/minimum_io_size
queue/optimal_io_size
queue/physical_block_size

on tool side,
for util-linux
ldd /sbin/fdisk | grep blkid
strings /lib/libblkid.so.1 | grep queue/

for lvm
strings /sbin/lvm |grep queue/

and so on.

-- 
Luca Berra -- bluca@comedia.it

Re: [dm-crypt] status of dm-crypt (alignment issues) on a SSD

[Milan Broz]

On 12/02/2010 08:54 AM, Luca Berra wrote:

> what you need on kernel side is having in sysfs
> under /sys/block/<device>
> queue/minimum_io_size
> queue/optimal_io_size
> queue/physical_block_size

Just not all SSD presents proper values here.

My advice is align to 1MiB in such case and it will work:-)
(this is default in recent versions - not all yet in Debian)

You can always do it manually, use fdisk -u (to work in sector mode,
note sector is 512B. Switch off DOS mode if it is enabled - there
should be warning)

For lvm, use pvcreate --dataalignment, then check with pvs -o +pe_start.
(You can use --units S to work in sectors mode like in fdisk.
2048 sectors == 1MiB, any multiple of it is ok)

(See man page and check archive, I think it was described here
several times.)

Milan

Re: [dm-crypt] status of dm-crypt (alignment issues) on a SSD

[mike dentifrice]

Thanks a lot for your reply!

Luca Berra a dit:
> what you need on kernel side is having in sysfs
> under /sys/block/<device>
> queue/minimum_io_size
> queue/optimal_io_size
> queue/physical_block_size

These are present while running Debian's 2.6.32-5 linux kernel.

> on tool side,
> for util-linux
> ldd /sbin/fdisk | grep blkid

$ ldd /sbin/fdisk | grep blkid
        libblkid.so.1 => /lib/libblkid.so.1 (0x00007fd182e63000)

> strings /lib/libblkid.so.1 | grep queue/

$ strings /lib/libblkid.so.1 | grep queue/
	queue/minimum_io_size
	queue/optimal_io_size
	queue/physical_block_size

> for lvm
> strings /sbin/lvm |grep queue/

$ strings /sbin/lvm | grep queue/
	queue/optimal_io_size
	queue/minimum_io_size

> and so on.

So far, everything seems good as far as I understnad, but should
`cryptsetup` have 'queue' strings as well ? Mine (v1.1.3) doesn't. I
compiled 1.2.0-rc1 and it doesn't either. Is that a stopper of not?

Thanks,

-- 
mike dentifrice <fluor@poivron.org>

Re: [dm-crypt] status of dm-crypt (alignment issues) on a SSD

[Milan Broz]

On 12/02/2010 12:50 PM, mike dentifrice wrote:
> So far, everything seems good as far as I understnad, but should
> `cryptsetup` have 'queue' strings as well ? Mine (v1.1.3) doesn't. I
> compiled 1.2.0-rc1 and it doesn't either. Is that a stopper of not?

Grepping for queue is not clever idea, sorry.

1.1.3 has topology support, 1.2.0 adds 1MiB default here.

Run luksFormat --debug and you will see topology info read from kernel:

# Topology: IO (512/0), offset = 0; Required alignment is 4096 bytes.

512/0, offset = 0 means min io size / opt io size and offset from sysfs.

For ssd it should be surenly not 512 but something like write block.

Milan