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RAID 5 DISKS

Hi,
How do we count the total physical disks assigned to a DB2 database
on a RAID 5, solaris environment.

Thank you.

May 22 '07 #1
8 1948
vishnu wrote:
Hi,
How do we count the total physical disks assigned to a DB2 database
on a RAID 5, solaris environment.

Thank you.
Raid 5 = (n-1) DATA disk and PARITY disk. So if I have $aid5 array on 5
disk - I would consider that my DB2 has 4 Data disks.

Jan M. Nelken

May 22 '07 #2
vishnu wrote:
Hi,
How do we count the total physical disks assigned to a DB2 database
on a RAID 5, solaris environment.

Thank you.

I would consider this as:

Raid 5 on n disks = (n-1) DATA disk and PARITY disk.

So if I have $aid5 array on 5 disk - I would consider that my DB2 has 4
Data disks. OI know that parity chunk alternates between all disks - but
1 logical i/o operation activates 4 writes of data + 1 write of parity.

Jan M. Nelken

May 22 '07 #3
What cmd do I run to check the number of disks I have.

Jan M. Nelken wrote:
>Hi,
How do we count the total physical disks assigned to a DB2 database
on a RAID 5, solaris environment.

Thank you.

I would consider this as:

Raid 5 on n disks = (n-1) DATA disk and PARITY disk.

So if I have $aid5 array on 5 disk - I would consider that my DB2 has 4
Data disks. OI know that parity chunk alternates between all disks - but
1 logical i/o operation activates 4 writes of data + 1 write of parity.

Jan M. Nelken
May 22 '07 #4
On May 22, 10:27 am, "vishnu" <u34415@uwewrote:
What cmd do I run to check the number of disks I have.

Jan M. Nelken wrote:
Hi,
How do we count the total physical disks assigned to a DB2 database
on a RAID 5, solaris environment.
Thank you.
I would consider this as:
Raid 5 on n disks = (n-1) DATA disk and PARITY disk.
So if I have $aid5 array on 5 disk - I would consider that my DB2 has 4
Data disks. OI know that parity chunk alternates between all disks - but
1 logical i/o operation activates 4 writes of data + 1 write of parity.
Jan M. Nelken- Hide quoted text -

- Show quoted text -
For most purposes in DB2, a single arrary of 5 disks (or any number of
disks in the array) should be considered by a DBA to be one large
physical disk. But it may depend on why you asking the question.

May 26 '07 #5
Mark A wrote:
For most purposes in DB2, a single arrary of 5 disks (or any number of
disks in the array) should be considered by a DBA to be one large
physical disk. But it may depend on why you asking the question.
Right - for almost all purposes DB2 should be told that there is one
disk there.

But - sometimes it is beneficial to know number of data disks in array -
as well as physical raid stripe size - in order to calculate efficient
extend size and/or prefetch size.

Jan M. Nelken
May 26 '07 #6
On May 25, 9:58 pm, "Jan M. Nelken" <Unknown.U...@Invalid.Domain>
wrote:
Mark A wrote:
For most purposes in DB2, a single arrary of 5 disks (or any number of
disks in the array) should be considered by a DBA to be one large
physical disk. But it may depend on why you asking the question.

Right - for almost all purposes DB2 should be told that there is one
disk there.

But - sometimes it is beneficial to know number of data disks in array -
as well as physical raid stripe size - in order to calculate efficient
extend size and/or prefetch size.

Jan M. Nelken
Any time you are dealing with striped disks, the extent size should be
an even multiple of stripe size.

But for purposes of prefetch size, a single array should be considered
as one disk. When creating containers, if you follow the rule that for
a given tablespace there should only be one container per physical
disk, then if you have a single striped array of 5 disks, you should
consider that to be one disk. Since the prefetch size should be the
extent size, times the number of containers in the tablespace, then
the extent size would equal the prefetch size if only one array is
used (and one container is created for that tablespace in the single
array).

May 26 '07 #7
Mark A wrote:
Any time you are dealing with striped disks, the extent size should be
an even multiple of stripe size.

But for purposes of prefetch size, a single array should be considered
as one disk. When creating containers, if you follow the rule that for
a given tablespace there should only be one container per physical
disk, then if you have a single striped array of 5 disks, you should
consider that to be one disk. Since the prefetch size should be the
extent size, times the number of containers in the tablespace, then
the extent size would equal the prefetch size if only one array is
used (and one container is created for that tablespace in the single
array).
Setting extend size = prefetch size for that case you described above
would turn off prefetching, wouldn't it?

Jan M. Nelken

May 26 '07 #8
On May 26, 11:01 am, "Jan M. Nelken" <Unknown.U...@Invalid.Domain>
wrote:
Setting extend size = prefetch size for that case you described above
would turn off prefetching, wouldn't it?

Jan M. Nelken
Prefetch takes place when DB2 decides to put pages into the bufferpool
asynchronously before the pages are actually needed by DB2. In that
sense itt is based on an artificial intellignece algorithm. This
typically happens in a table scan of a large table where DB2 knows
that all the pages of the table will eventually need to be in the
bufferpool. But DB2 needs to make sure that prefetching is not too
fast if the bufferpool does not have room for all the pages, otherwise
previously prefetched pages could be flushed out before they are
actually used for the query.

Prefetching can certainly happen if prefetch size equals extent size.
DB2 will just prefetch one extent at a time (assuming that prefetch is
turned on and DB2 thinks it is a good idea for a specific query). The
reason for making the prefetch size equal to the extent size times
number of containers is that otherwise you will have disk contention
if you are prefetchng multiple extents from the same disk or array at
the same time. That is why you should generally only use multiple
containers if they can be placed on different disks or different
arrays.

May 26 '07 #9

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