chouinar@centrcn.umontreal.ca (Luc Chouinard) (05/11/91)
We have 6 scsi disks on two controlers, and we are
considering using a striped logical volume for one
of our partitions.
This logical volume would be spread across four disks, two
on each controler.
We estimate that about 50 users will access this partition
at the same time.
We currently believe that striping is always a good thing
to do. It distributes uniformely the disk access throughout
the controlers.
We have just been advised by a knowledgeable person that
this would not be true in all cases, specifically ours.
And the explanation that was given to us makes sense.
We would appreciate your opinions on the matter of using
striped volumes, according to the different environments.
More specifically, we would like to know if there is an
overhead associated with accessing a single block in a
striped logical volume, versus a standard partition.
Any comment will be appreciated.
Thank you.
--
+---------------------------------------------------------+
| Andre Paquet Analyst University of Montreal +
| Computer Center paqueta@jsp.umonteal.ca +
+---------------------------------------------------------+olson@anchor.esd.sgi.com (Dave Olson) (05/12/91)
In <9105101949.AA20619@centrs1.CENTRCN.UMontreal.CA> chouinar@centrcn.umontreal.ca (Luc Chouinard) writes: | We have 6 scsi disks on two controlers, and we are | considering using a striped logical volume for one | of our partitions. | | This logical volume would be spread across four disks, two | on each controler. | | We estimate that about 50 users will access this partition | at the same time. | | We currently believe that striping is always a good thing | to do. It distributes uniformely the disk access throughout | the controlers. | | We have just been advised by a knowledgeable person that | this would not be true in all cases, specifically ours. | And the explanation that was given to us makes sense. | | We would appreciate your opinions on the matter of using | striped volumes, according to the different environments. | More specifically, we would like to know if there is an | overhead associated with accessing a single block in a | striped logical volume, versus a standard partition. There is of course SOME overhead in using logical volumes. It is quite low (lost in the noise for almost all the tests we benchmarked). Since you are running on 2 controllers, you should see a net gain for almost all cases. About the only time IRIX will read a single block from a file that is > 1 block long is when doing lots of 'random' i/o, and during directory searches if the directories are small. I'd be interested in the knowledgable person's arguments, and whether they are based on measurements on SGI or other implementations of striped disks. -- Dave Olson Life would be so much easier if we could just look at the source code.
daveh@xtenk.asd.sgi.com (David Higgen) (05/14/91)
In article <9105101949.AA20619@centrs1.CENTRCN.UMontreal.CA>, chouinar@centrcn.umontreal.ca (Luc Chouinard) writes: > We have 6 scsi disks on two controlers, and we are > considering using a striped logical volume for one > of our partitions. > > We currently believe that striping is always a good thing > to do. > > We have just been advised by a knowledgeable person that > this would not be true in all cases, specifically ours. > And the explanation that was given to us makes sense. Would you care to mail me this explanation? I would like to see if it makes sense to me & may be able to advise you better: I am the author of SGI's logical volume implementation. > More specifically, we would like to know if there is an > overhead associated with accessing a single block in a > striped logical volume, versus a standard partition. The overhead is negligible: a sufficiently small fraction of a percent that we have not been able to detect it in practical performance measurements. Dave Higgen (daveh@xtenk.asd.sgi.com)