yes, this can get complicated... Yes, multiple threads accessing different
volumes on the same spindles can create head contention, even with volumes
formatted serially. But I think you can still reap benefits from laying down
blocks sequentially on the filesystem. Remco points out read-ahead benefits,
and he is (IMHO) referring to disk array-based read-ahead. Keep in mind that
jfs[2] also has read-ahead, and it will still try to do this regardless of
whether the physical blocks are laid down sequentially - it will just result in
more head movement, more latency, and less efficiency. I do not believe that
jfs2 read-ahead uses array-based read-ahead. The array-based read-ahead will
pre-stage blocks in array cache, whereas jfs2-based read-ahead will pre-stage
them in jfs mbufs.
When the array is doing read-ahead, it will turn a single-block read into a
multi-block read. Since the blocks are laid down in sequence, there will be (I
think) less head contention during this array-based read-ahead. Not the case
for jfs2 read-ahead.
not to get lost: preformatting volumes ahead of time and not letting them get
scratched and re-created on-demand will avoid filesystem fragmentation and
randomization of the blocks. It's too bad that TSM can't manage pre-formatted
volumes as scratch volumes that can be shared between different storage pools
or even different servers (managed by the shared library manager of course).
..Paul (with Holiday Inn disclaimer)
At 03:01 PM 10/20/2010, Richard Rhodes wrote:
>This can get complicated.
>
>File devices, as Paul states, are mostly accessed sequentially.
>But, as has been also said, the actual file volumes may be fragmented on
>the filesystem, resulting is effective random access.
>But, also, TSM may/probably will be accessing multiple file devices
>concurrently. This can also result is effective random access.
>But, also, also, if you are using a disk array you also need to take into
>consideration the lun layout.. Most big disk arrays share spindles among
>multiple servers (wide stripping).
>
>Unless you have a a single tsm task accessing a single file device (that is
>not fragmented) on a dedicated disk, then there will contention for I/O's.
>
>Rick
>
>
>
>
>
>
> Paul Zarnowski
> <psz1 AT CORNELL DOT EDU
>
> > To
> Sent by: "ADSM: ADSM-L AT VM.MARIST DOT EDU
>
> Dist Stor cc
> Manager"
> <[email protected] Subject
> .EDU> Re: Lousy performance on new
> 6.2.1.1 server with
> SAN/FILEDEVCLASS storage
> 10/20/2010 02:19
> PM
>
>
> Please respond to
> "ADSM: Dist Stor
> Manager"
> <[email protected]
> .EDU>
>
>
>
>
>
>
>I/O to devclass file volumes will be inherently sequential, yes. It's not
>an absolute, however. There are varying degrees of "sequentialness".
>Think about it this way. When you are writing these volumes, they will
>definitely be purely sequential. However, when reading them, they may or
>may not be purely sequential. If you are only restoring say "active"
>backup files, then TSM would be skipping over the "inactive" files that can
>be interspersed between the active files. Yes, they may have been "active"
>when they were written (depending on what did the writing - client or
>migration), but by the time you go to read the data, depending on what is
>doing the reading you may not be reading them purely sequentially. Even if
>you are not reading them purely sequentially, I believe you will still
>likely reap benefits by having the blocks laid down on disk sequentially.
>Note than when I say laid down on disk sequentially, this includes the idea
>of striping the blocks across spindles (if you are doing striping).
>Striping does not defeat the sequentiality.
>
>..Paul
>
>At 02:11 PM 10/20/2010, Hart, Charles A wrote:
>>Dumb statement, but isn't the whole Idea of the File Devclass is it is
>sequential. Can one be more sequential than the other? If its not then
>its random.
>>
>>
>>-----Original Message-----
>>From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf
>>Of
>Paul Zarnowski
>>Sent: Wednesday, October 20, 2010 1:07 PM
>>To: ADSM-L AT VM.MARIST DOT EDU
>>Subject: Re: [ADSM-L] Lousy performance on new 6.2.1.1 server with
>SAN/FILEDEVCLASS storage
>>
>>How you connect to the disk storage (i.e., SCSI or SAN) doesn't matter.
>This goes more to the issue of how blocks within the volumes are laid out
>on the spindles. formatting them one at a time will cause the blocks to be
>laid out in a more sequential fashion, so that when TSM references the
>blocks, they will be referenced in a more sequential fashion (assuming you
>are doing mostly sequential I/O).
>>
>>..Paul
>>
>>
>>At 02:02 PM 10/20/2010, Zoltan Forray/AC/VCU wrote:
>>>Thanks for the affirmation. This is what I have been
>seeing/experiencing.
>>> As soon as I can empty the stgpool (5TB), I will define fixed volumes
>and
>>>see how much difference that makes. I am aware of the issue of
>>>single-threading the define/formats to not fragment them, however I
>>>wonder how much that really matters in a SAN?
>>>Zoltan Forray
>>>TSM Software & Hardware Administrator
>>>Virginia Commonwealth University
>>>UCC/Office of Technology Services
>>>zforray AT vcu DOT edu - 804-828-4807
>>>Don't be a phishing victim - VCU and other reputable organizations will
>>>never use email to request that you reply with your password, social
>>>security number or confidential personal information. For more details
>>>visit http://infosecurity.vcu.edu/phishing.html
>>>
>>>
>>>
>>>From:
>>>Markus Engelhard <markus.engelhard AT BUNDESBANK DOT DE>
>>>To:
>>>ADSM-L AT VM.MARIST DOT EDU
>>>Date:
>>>10/20/2010 09:20 AM
>>>Subject:
>>>[ADSM-L] Lousy performance on new 6.2.1.1 server with SAN/FILEDEVCLASS
>>>storage Sent by:
>>>"ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU>
>>>
>>>
>>>
>>>Hi Zoltan,
>>>
>>>my experience has been: use fixed size preformatted volumes, and be
>>>sure to format them sequentially, even if it seems to take a hell of a
>>>time. But then, it´s a one-time action and highly automated, so just
>>>don´t try to boost "performance" here. Make sure no one else is bogging
>>>perfs, SAN guys sometimes tend to put all kinds of unassorted loads on
>>>one storage array producing massive hot-spots during TSM activities.
>>>
>>>Kind regards,
>>>
>>>Markus
>>>
>>>
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>>--
>>Paul Zarnowski Ph: 607-255-4757
>>Manager, Storage Services Fx: 607-255-8521
>>719 Rhodes Hall, Ithaca, NY 14853-3801 Em: psz1 AT cornell DOT edu
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>Paul Zarnowski Ph: 607-255-4757
>Manager, Storage Services Fx: 607-255-8521
>719 Rhodes Hall, Ithaca, NY 14853-3801 Em: psz1 AT cornell DOT edu
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