Why set read policy to "first" for ufsdump of mirror on Sol10?

Why set read policy to "first" for ufsdump of mirror on Sol10?

Post by Matt Sienkiewi » Thu, 18 Aug 2005 20:46:25



In the Solaris Volume Manager Administration Guide (in the
Solaris 10 System Administrator Collection),
http://docs.sun.com/app/docs/doc/816-4520/6manpieqb?a=view
it states:

"You can use the ufsdump command to backup the files of a mounted
filesystem residing on a RAID-1 volume. Set the read policy on the volume
to "first" when the backup utility is ufsdump. This improves the rate at
which the backup is performed."

 And

"After the ufsdump command is done, set the read policy on the mirror to
"roundrobin.""

This may have been the same for previous versions of Solaris and I just
never noticed, but my question is why would this improve the rate?  Why is
it faster/better to limit reads to the first disk?

Matt S

 
 
 

Why set read policy to "first" for ufsdump of mirror on Sol10?

Post by Jorgen Moquis » Sat, 20 Aug 2005 09:05:09



> In the Solaris Volume Manager Administration Guide (in the
> Solaris 10 System Administrator Collection),
> http://docs.sun.com/app/docs/doc/816-4520/6manpieqb?a=view
> it states:

> "You can use the ufsdump command to backup the files of a mounted
> filesystem residing on a RAID-1 volume. Set the read policy on the volume
> to "first" when the backup utility is ufsdump. This improves the rate at
> which the backup is performed."

>  And

> "After the ufsdump command is done, set the read policy on the mirror to
> "roundrobin.""

> This may have been the same for previous versions of Solaris and I just
> never noticed, but my question is why would this improve the rate?  Why is
> it faster/better to limit reads to the first disk?

> Matt S

my guess:
  most modern disks reads the whole track, even requesting one sector.
  next sector is then from the buffer/cache not another physical read.

  so next sector is already read by that disk, available without a seek.

  why its recommended to use round robin as default is not clear to me,
  other than the system spots a faulty disk better.

  read first seems to me to be the most efficient conf.
  at all times as i see it.

/J?rgen

 
 
 

Why set read policy to "first" for ufsdump of mirror on Sol10?

Post by Darren Dunha » Wed, 24 Aug 2005 04:57:49



> my guess:
>   most modern disks reads the whole track, even requesting one sector.
>   next sector is then from the buffer/cache not another physical read.
>   so next sector is already read by that disk, available without a seek.
>   why its recommended to use round robin as default is not clear to me,
>   other than the system spots a faulty disk better.

I would believe that there are many disks out there that have top speeds
well below that of the system bus.  As such, 'first' would limit your
throughput to that of one disk.  round robin should help if you are
operating near that limit and above.  It's something to try.
Prefetching the whole track would certainly limit this if it occurs.  I
don't know that I've seen the result of that occuring before.

In addition, you may have a database with very random access patterns,
most of which are to a single block.  There the ability to have the two
disks seek independently gives increased performance.  This situation is
probably more rare the the other.

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1. Why so many ufsdump processes after I type "ufsdump 0f /dev/null /"?

ptree shows:
            1142  -ksh
              3606  ufsdump 0f /dev/null /
                3607  ufsdump 0f /dev/null /
                  3608  ufsdump 0f /dev/null /
                  3609  ufsdump 0f /dev/null /
                  3610  ufsdump 0f /dev/null /
                  3611  ufsdump 0f /dev/null /
And it seems that only one ufsdump is working and the rest are sleeping.
Why then do we need all of the ufsdump processes? Thanks.

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7. Creating mirrored machine with "ufsdump and ufsrestore"

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12. part 2/2 of: running ufsdump under "script ufsdump.log"

13. part 1/2 of: running ufsdump under "script ufsdump.log"