Re: Disk partition alignment

----- Original Message ----- 
From: "Marcos Felipe Rasia de Mello" <[email protected]>
To: <[email protected]>
Sent: Sunday, April 17, 2011 4:28 AM
Subject: [IPCop-devel] Disk partition alignment


> The current parted commands used by the installler are
>
> parted -s $dev mklabel msdos
> parted -s $dev mkpart primary 63s ${root_size}MiB
> parted -s $dev mkpart primary ${root_size}MiB ${disk_size}MiB
> parted -s $dev set 1 boot on
>
> A comment in the disk-partition.sh script says "disks are optimized
> for win that had the habit until XP to start first partition at 63s".
> I think this is not correct.
the comment is correct but the behavior has changed since XP.

> The main reason why Vista started putting
> the first partition at sector 2048 was to get good performance with
> 4KB physical sector hard disks. Parted started doing the same thing by
> default (for all partitions) since version 2.1 as far I remember.
>
I had think to align to sector 2048 but haven't yet made.

> With "mkpart primary 63s ${root_size}MiB" and "mkpart primary
> ${root_size}MiB ${disk_size}MiB" IPcop will use an odd partitionig
> scheme. The first partition will be missaligned with 4KB sector HDs.
> *But* the second partition will be aligned, because is the default
> behaviour of parted when the sector unit isn't used. This will bring
> very bad performance with Western Digital drives with "Advance Format"
> [1] (all models ending with "EARS") in the first parition, but not in
> the second!
>
> The Seagate 4KB drives are smarter, because they realize when the
> partition is misaligned an compensate it (I have no clue how it works
> internally) in the firmware (they call "SmartAlign") [2]. But I have
> doubts if it work with one partiton aligned and other not.
>
thank for that information.
I read TP615 that send you to MB604 document [3]
Mostly, that say you have to use VISTA sp1 or use Seagate Advanced Format
drive with Smartalign Technology or use a partition program that align
partition.

I suppose new seagate firmware is smart enough to align partition limit to
4kb sector boundaries.

> The right way to force the old aligment is parted's "-a cylinder"
> option. With the option, start of the first partition can be
> represented by 0% and parted will put it in sector 63.
>
Why not simply use the 2048 sector shift?

> Maybe is a good idea IPcop fallow all major Linux distributions. All
> of them use the MiB aligment now. Just replace "63s" by "0%" and it's
> done. But there are reports of very broken BIOSes that refuses to boot
> with the MiB alignment. Taking into account that IPcop can run very
> well with old PCs (more likely to has buggy BIOS), can be dangerous..
>
Anyway, we should think aligment requirement with SSD in mind.
That would mean we may require an advanced setting with aligment on erase
block boudaries (that vary greatly with disk model). I don't know if we have
a way to read the erase block size somewhere.

> Reference about 4KB physical sector HDs and Linux:
> https://ata.wiki.kernel.org/index.php/ATA_4_KiB_sector_issues
>
> I say this based on x86 PCs only. With other architectures I don't
> know anything :)
>
Actually, pcc was broken with hardened gcc patch.
sparc was too broken there but I think it need another fix (compile gcc
against newlib to compile a 64bits kernel properly)
I haven't yet tried to fix that.

> [1] http://www.wdc.com/global/products/features/?id=7&language=1
> [2]
http://www.seagate.com/docs/pdf/whitepaper/tp615_smartalign_for_af_4k.pdf
>
    [3]
http://www.seagate.com/docs/pdf/whitepaper/mb604_4k_transition_faq.pdf

Gilles


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