[arch-general] GPT slower than MBR, although both are properly aligned?

[Karol Babioch]

Hi,

I'm about to setup a new Arch box and wanted to use GPT instead of the
(old) MBR partition scheme, just for the fun of it ;).

I've read through the various articles in the wiki, but it seems that
something strange is going on here.

I'm about to install the system to a SSD (Kingston S100), therefore I
also have to consider proper alignment. I've created the partition
scheme with gdisk (alignment set to 2048 boundary, although I've tried
it with different values (up to 8192), too), which basically looks like
that:

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048          264191   128.0 MiB   0700  Linux/Windows data
   2          264192        31277198   14.8 GiB    0700  Linux/Windows data

Take a look at the start sectors, they should be properly aligned to
2048, as 264192 is dividable by 2048.

In order to check the proper alignment I've run hdparm -tT over both
partitions, here are the results:

/dev/sda1:
 Timing cached reads:   12026 MB in  2.00 seconds = 6017.15 MB/sec
 Timing buffered disk reads: 128 MB in  0.64 seconds = 201.56 MB/sec

/dev/sda2:
 Timing cached reads:   12086 MB in  2.00 seconds = 6047.19 MB/sec
 Timing buffered disk reads: 278 MB in  3.02 seconds =  92.02 MB/sec

Quite a huge discrepancy considering the fact that a SSD should have
roughly the same speed everywhere.

Well, after fiddling around with it a little bit, I've finally tried the
same setup with the old MBR partition scheme, using fdisk, which gave me
the following partition scheme:

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048      264191      131072   83  Linux
/dev/sda2          264192    31277231    15506520   83  Linux

So the partition scheme is the same, despite the fact that the second
partition is a little bit bigger with MBR as there is no copy of the
partition table.

With the MBR partitions I get the following results using hdparm -tT in
order to measure the speed:

/dev/sda1:
 Timing cached reads:   11578 MB in  2.00 seconds = 5793.15 MB/sec
 Timing buffered disk reads: 128 MB in  0.63 seconds = 202.22 MB/sec

/dev/sda2:
 Timing cached reads:   11896 MB in  2.00 seconds = 5952.79 MB/sec
 Timing buffered disk reads: 604 MB in  3.00 seconds = 201.16 MB/sec

So, both partitions are, as to be expected, equal in speed.

I'm quite astonished by these results, which confuse me totally. Is it
possible that GPT is slower than MBR? Is there something wrong
implemented within gdisk? Is there any explanation to that?

Right now the only thing I can do to get full speed is to use MBR once
again :(.

Best regards,
Karol Babioch


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