XFS peculiar behavior

[Yannis Klonatos]

Hello again,

         First of all, thank you all for your quick replies. I attach 
all the information you requested in your responses.

1) The output of xfs_info is the following:

meta-data=/dev/sdf             isize=256    agcount=32, agsize=45776328 blks
                =                         sectsz=512   attr=0
data         =                         bsize=4096   blocks=1464842496, 
imaxpct=25
                =                         sunit=0      swidth=0 blks, 
unwritten=1
naming    =version 2            bsize=4096
log          =internal               bsize=4096   blocks=32768, version=1
               =                          ectsz=512   sunit=0 blks, 
lazy-count=0
realtime  =none                   extsz=4096   blocks=0, rtextents=0

2) The output of xfs_bmap in the lineitem.MYI table of the TPC-H 
workload is at one run:

/mnt/test/mysql/tpch/lineitem.MYI:
  EXT: FILE-OFFSET           BLOCK-RANGE              AG 
AG-OFFSET          TOTAL
    0: [0..6344271]:         11352529416..11358873687 31 
(72..6344343)    6344272
    1: [6344272..10901343]:  1464842608..1469399679    4 
(112..4557183)   4557072
    2: [10901344..18439199]: 1831053200..1838591055    5 
(80..7537935)    7537856
    3: [18439200..25311519]: 2197263840..2204136159    6 
(96..6872415)    6872320
    4: [25311520..26660095]: 2563474464..2564823039    7 
(96..1348671)    1348576

Given that all disk blocks are in units of 512-byte blocks, if I 
interpret the output
correctly the first file is at block 1465352792 = 698.4GByte offset and 
the last block
is at 5421.1GByte offset, meaning that this specific table is split over 
a 4,7TByte distance.

However, in another run (with a clean file system again)

/mnt/test/mysql/tpch/lineitem.MYI:
  EXT: FILE-OFFSET      BLOCK-RANGE              AG AG-OFFSET           
TOTAL
    0: [0..26660095]:   11352529416..11379189511 31 (72..26660167)   
26660096

3) For the copy, as i mentioned in my previous mail, i copied the 
database over nfs using the cp -R linux program.
Thus, i believe all the files are copied sequentially, the one after the 
other, with no other concurrent write operations
running at the background. The file-system was pristine before the cp 
with no files, and just the mount directory was
created (all the other necessary files and directories are created from 
the cp program).

4)  The version of xfsprogs is 2.9.4 (acquired with xfs_info -v) and the 
version of the kernel is 2.6.18-164.11.1.el5.

         If you require any further information let me know. Let me 
state that i can also provide you with the complete
data-set if you feel it necessary trying to reproduce the issue.

Thanks,
Yannis Klonatos
>> Hi all!
>>
>>          I have come across the following peculiar behavior in XFS
>> and i would appreciate any information anyone
>> could provide.
>>          In our lab we have a system that has twelve 500GByte hard
>> disks (total capacity 6TByte), connected to an
>> Areca (ARC-1680D-IX-12) SAS storage controller. The disks are
>> configured as a RAID-0 device. Then I create
>> a clean XFS filesystem on top of the raid volume, using the whole
>> capacity. We use this test-setup to measure
>> performance improvement for a TPC-H experiment. We copy the database
>> over the clean XFS filesystem using the
>> cp utility. The database used in our experiments is 56GBytes in size
>> (data + indices).
>>          The problem is that i have noticed that XFS may - not all
>> times - split a table over a large disk distance. For
>> example in one run i have noticed that a file of 13GByte is split
>> over a 4,7TByte distance (I calculate this distance
>> by subtracting the final block used for the file with the first one.
>> The two disk blocks values are acquired using the
>> FIBMAP ioctl).
>>          Is there some reasoning behind this (peculiar) behavior? I
>> would expect that since the underlying storage is so
>> large, and the dataset is so small, XFS would try to minimize disk
>> seeks and thus place the file sequentially in disk.
>> Furthermore, I understand that there may be some blocks left unused
>> by XFS between subsequent file blocks used
>> in order to handle any write appends that may come afterward. But i
>> wouldn't expect such a large splitting of a single
>> file.
>>          Any help?
>>