How to verify that 4K blocks are properly set up with ZFS?

[ajm83]

How do I check that 4K blocks are properly set up for an existing pool? The drives are 4k but report 512 as the sector size. My ashift reported by zdb|grep ashift is 12, but that is not enough to determine that everything is working according to @Sebulon's post here: viewtopic.php?p=174638#p174638.

I inherited the pool after somebody else created it so I don't know exactly how it was set up.

Here is the info of one of my disks:

Diskinfo output from one disk:

# diskinfo -v ada0
ada0
        512             # sectorsize
        3000592982016   # mediasize in bytes (2.7T)
        5860533168      # mediasize in sectors
        4096            # stripesize
        0               # stripeoffset
        5814021         # Cylinders according to firmware.
        16              # Heads according to firmware.
        63              # Sectors according to firmware.
        WD-WMC1T0974497 # Disk ident.

Gpart output from the same disk:

# gpart list ada0
Geom name: ada0
modified: false
state: OK
fwheads: 16
fwsectors: 63
last: 5860533134
first: 34
entries: 128
scheme: GPT
Providers:
1. Name: ada0p1
   Mediasize: 262144 (256k)
   Sectorsize: 512
   Stripesize: 4096
   Stripeoffset: 1024
   Mode: r0w0e0
   rawuuid: 89d9f7ef-92dd-11e2-a4a3-6805ca115e16
   rawtype: 83bd6b9d-7f41-11dc-be0b-001560b84f0f
   label: (null)
   length: 262144
   offset: 17408
   type: freebsd-boot
   index: 1
   end: 545
   start: 34
2. Name: ada0p2
   Mediasize: 3000590401536 (2.7T)
   Sectorsize: 512
   Stripesize: 4096
   Stripeoffset: 0
   Mode: r1w1e2
   rawuuid: 89f60ed7-92dd-11e2-a4a3-6805ca115e16
   rawtype: 516e7cba-6ecf-11d6-8ff8-00022d09712b
   label: bdldisk0
   length: 3000590401536
   offset: 1048576
   type: freebsd-zfs
   index: 2
   end: 5860530175
   start: 2048
Consumers:
1. Name: ada0
   Mediasize: 3000592982016 (2.7T)
   Sectorsize: 512
   Stripesize: 4096
   Stripeoffset: 0
   Mode: r1w1e3

There is both 512 and 4096 in the gpart output from the disks, I'm really confused about how to tell now what is going on. I have read the manpage but I am none the wiser as to what sectorsize, stripesize and stripeoffset actually mean.

Raw disk performance of the pool seems poor (6 disks in striped mirror configuration) which is why I am investigating this.

Any clues welcomed, thanks!

 

[wblock@]

Re: How to verify that 4K blocks are properly set up with ZF

Please show the output of gpart show to see if the partitions are aligned. ashift being 12 means that ZFS is using 4K blocks, which is good. If the alignment is also correct, all is well.

 

[ajm83]

Re: How to verify that 4K blocks are properly set up with ZF

A, of course, thanks @wblock@ and thanks to the moderator for fixing the formatting of my post.

gpart show ada0
=>        34  5860533101  ada0  GPT  (2.7T)
          34         512     1  freebsd-boot  (256k)
         546        1502        - free -  (751k)
        2048  5860528128     2  freebsd-zfs  (2.7T)
  5860530176        2959        - free -  (1.5M)

Is 2048 the correct starting point for that partition? It seems like it having read other posts.

 

[wblock@]

Re: How to verify that 4K blocks are properly set up with ZF

Yes, 2048*512 = 1M, an even multiple of 4K. Or an easier way of working that backward is to divide the number of 512-byte blocks by eight. If the result is an integer, the number was an even multiple of 4K.

2048/8 = 256. Now for the size: 5860528128/8 = 732566016. So the both the start and size are an even multiple of 4K. It's good.

 

[ajm83]

Re: How to verify that 4K blocks are properly set up with ZF

Yes, 2048*512 = 1M, an even multiple of 4K. Or an easier way of working that backward is to divide the number of 512-byte blocks by eight. If the result is an integer, the number was an even multiple of 4K.

2048/8 = 256. Now for the size: 5860528128/8 = 732566016. So the both the start and size are an even multiple of 4K. It's good.

That's excellent, thank you very much @wblock@. :beer