alignment of metadata 1.2 and mkfs.ext4

Hi all (well, Neil, really),

The the following, Neil states that that is no issue with alignment of
metadata 1.2 and mkfs.ext4 on 4K drives:

http://www.issociate.de/board/post/507176/filesystem_on_mdad m_raid_from_unpartioned_block_devices.html

Sorry to ask again, but I just want to clarify. Metadata 1.2 puts the
data 4K from the start of the device/partition. Does mkfs put the start
of the filesystem at 64K from the start? IE, will the following align,
without any need for options?

(create partitions starting 2048 512-byte sectors from start = 1MB)
mdadm --create /dev/md0 -v --raid-devices=4 --level=raid10
--metadata=1.2 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1
mkfs.ext4 /dev/md0

And, slightly off-topic, it's not necessary to define stride and stripe
any longer?
mkfs.ext4 -E stride=16,stripe-width=32 /dev/md0

Regards,
Tyler


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Re: alignment of metadata 1.2 and mkfs.ext4

Hi all,

like Tyler I'm also asking me those questions.
Reason is that newer Solid-State Drives (like the Intel 320 Series) have
Flash-Chips with page size of 8KiB (the former SSDs have mostly 4KiB
pages). So in the best case, all data written to such an SSD should be
at least 8KiB aligned - which I think is not the case for MD-RAID with
metadata 1.2 superblocks - because of the "4K from the beginning of the
device":
https://raid.wiki.kernel.org/index.php/RAID_superblock_forma ts#Sub-versions_of_the_version-1_superblock
When I understand the wiki page correctly, the metadata 1.2 superblock
has a size of 1024 Bytes (256 Bytes + 2 Bytes/device for a maximum of
384 devices), right?

When I simply put an Ext4 file system with a block size of 4KiB on the
RAID, there is no problem as there will always be two Ext4 blocks
residing in a single 8 KiB page of the SSD.

My questions:
1) But when I use LVM2 with a PE size of 4 MiB on top of a RAID, I will
not be perfectly aligned any more, right?
2) Are there any plans that e.g. the first 1MiB will be reserved for
metadata (like fdisk does when creating new partitions), so that
there won't be any alignment issues in the future?
(btw: haven the metadata at the end of the device like in the
metadata format 0.90 or 1.0 would also not interfere the alignment
- is there a special reason why the metadata has been put to the
beginning of the device?)

best regards,
Werner

On Wed, 2011-07-06 at 08:11 +0100, Tyler J. Wagner wrote:
> Hi all (well, Neil, really),
>
> The the following, Neil states that that is no issue with alignment of
> metadata 1.2 and mkfs.ext4 on 4K drives:
>
> http://www.issociate.de/board/post/507176/filesystem_on_mdad m_raid_from_unpartioned_block_devices.html
>
> Sorry to ask again, but I just want to clarify. Metadata 1.2 puts the
> data 4K from the start of the device/partition. Does mkfs put the start
> of the filesystem at 64K from the start? IE, will the following align,
> without any need for options?
>
> (create partitions starting 2048 512-byte sectors from start = 1MB)
> mdadm --create /dev/md0 -v --raid-devices=4 --level=raid10
> --metadata=1.2 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1
> mkfs.ext4 /dev/md0
>
> And, slightly off-topic, it's not necessary to define stride and stripe
> any longer?
> mkfs.ext4 -E stride=16,stripe-width=32 /dev/md0
>
> Regards,
> Tyler
>
>

--
: Werner Fischer
: Technology Specialist
: Thomas-Krenn.AG | The server-experts
: http://www.thomas-krenn.com | http://www.thomas-krenn.com/wiki

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Re: alignment of metadata 1.2 and mkfs.ext4

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On Wed, 06 Jul 2011 08:11:16 +0100
"Tyler J. Wagner" wrote:

> Metadata 1.2 puts the data 4K from the start of the device/partition.

This is not correct. See mdadm -E /dev/sdX.

$ sudo mdadm -E /dev/sdc3
/dev/sdc3:
....
Version : 1.2
....
Data Offset : 2048 sectors
....


--=20
With respect,
Roman

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Re: alignment of metadata 1.2 and mkfs.ext4

On Wed, 2011-07-06 at 15:20 +0600, Roman Mamedov wrote:
> On Wed, 06 Jul 2011 08:11:16 +0100
> "Tyler J. Wagner" wrote:
>
> > Metadata 1.2 puts the data 4K from the start of the device/partition.
>
> This is not correct. See mdadm -E /dev/sdX.
>
> $ sudo mdadm -E /dev/sdc3
> /dev/sdc3:
> ...
> Version : 1.2
> ...
> Data Offset : 2048 sectors
> ...

That is very interesting. The mdadm man page, and everything I've seen
online, all say "The different sub-versions store the superblock at
different locations on the device, either at the end (for 1.0), at the
start (for 1.1) or 4K from the start (for 1.2)."

Regards,
Tyler

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Re: alignment of metadata 1.2 and mkfs.ext4

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On Wed, 06 Jul 2011 10:52:34 +0100
"Tyler J. Wagner" wrote:

> That is very interesting. The mdadm man page, and everything I've seen
> online, all say "The different sub-versions store the superblock at
> different locations on the device, either at the end (for 1.0), at the
> start (for 1.1) or 4K from the start (for 1.2)."

And? The quote is about location of the mdadm superblock (metadata), not al=
ignment or offset of user data.

--=20
With respect,
Roman

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Re: alignment of metadata 1.2 and mkfs.ext4

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On Wed Jul 06, 2011 at 10:52:34AM +0100, Tyler J. Wagner wrote:

> On Wed, 2011-07-06 at 15:20 +0600, Roman Mamedov wrote:
> > On Wed, 06 Jul 2011 08:11:16 +0100
> > "Tyler J. Wagner" wrote:
> >=20
> > > Metadata 1.2 puts the data 4K from the start of the device/partition.
> >=20
> > This is not correct. See mdadm -E /dev/sdX.
> >=20
> > $ sudo mdadm -E /dev/sdc3
> > /dev/sdc3:
> > ...
> > Version : 1.2
> > ...
> > Data Offset : 2048 sectors
> > ...
>=20
> That is very interesting. The mdadm man page, and everything I've seen
> online, all say "The different sub-versions store the superblock at
> different locations on the device, either at the end (for 1.0), at the
> start (for 1.1) or 4K from the start (for 1.2)."
>=20
That's the RAID superblock at 4K, not the data.

Cheers,
Robin
--=20
___ =20
( ' } | Robin Hill |
/ / ) | Little Jim says .... |
// !! | "He fallen in de water !!" |

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Re: alignment of metadata 1.2 and mkfs.ext4

On Wed, 2011-07-06 at 16:02 +0600, Roman Mamedov wrote:
> And? The quote is about location of the mdadm superblock (metadata),
> not alignment or offset of user data.

Ah, I mistyped in my original email. However, you've shown me how to
find out where user data begins, which is what I needed to know. Thank
you.

Regards,
Tyler

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