Tdisk question

[Mary Zervos]

I have 500 cyl. of tdisk defined on one pack and 50 cyl. of tdisk
defined on another. One user used the 50 cyl. on the second pack and
about 24 users have used 215 cyl.(43%used) on the first pack. Why is it
that I can only allocate a 130 cyl. maximum tdisk on the first pack but
can then go ahead and allocate a 110 cyl. tdisk, instead of being able
to allocate *one* 240 cyl. tdisk?

Thanks for any help.

Mary Zervos
VM Systems Programmer
Binghamton University
***@binghamton.edu

[Mary Zervos]

Thanks Jim for the insight. I had forgotten about fragmentation.

Mary

Mary--What you're seeing is fragmentation. TDISK is allocated in one
whole piece to a user. Think of PC disk allocation that you see when
you run a defrag. If you have 1 allocated cylinder out in the middle
of an otherwise empty 100 cylinder TDISK space, you cannot allocate a
99 cyl TDISK. You would get that by the first user getting 50
cylinders and the 2nd user getting 1 cylinder. When the first user
detaches the 50 cyl. disk, the 2nd user still has the 1 cyl in the
middle. You are left with 99 cylnders in total free but you can only
allocate a max of 50 cylinders and 49 cylinders.
Jim

Post by Mary Zervos
I have 500 cyl. of tdisk defined on one pack and 50 cyl. of tdisk
defined on another. One user used the 50 cyl. on the second pack
and about 24 users have used 215 cyl.(43%used) on the first pack.
Why is it that I can only allocate a 130 cyl. maximum tdisk on the
first pack but can then go ahead and allocate a 110 cyl. tdisk,
instead of being able to allocate *one* 240 cyl. tdisk?
Thanks for any help.
Mary Zervos
VM Systems Programmer
Binghamton University

Jim Bohnsack
Cornell Univ.
[phone]

[Jim Bohnsack]

Mary--What you're seeing is fragmentation. TDISK is allocated in one whole
piece to a user. Think of PC disk allocation that you see when you run a
defrag. If you have 1 allocated cylinder out in the middle of an otherwise
empty 100 cylinder TDISK space, you cannot allocate a 99 cyl TDISK. You
would get that by the first user getting 50 cylinders and the 2nd user
getting 1 cylinder. When the first user detaches the 50 cyl. disk, the 2nd
user still has the 1 cyl in the middle. You are left with 99 cylnders in
total free but you can only allocate a max of 50 cylinders and 49 cylinders.

Jim

I have 500 cyl. of tdisk defined on one pack and 50 cyl. of tdisk defined
on another. One user used the 50 cyl. on the second pack and about 24
users have used 215 cyl.(43%used) on the first pack. Why is it that I can
only allocate a 130 cyl. maximum tdisk on the first pack but can then go
ahead and allocate a 110 cyl. tdisk, instead of being able to allocate
*one* 240 cyl. tdisk?
Thanks for any help.
Mary Zervos
VM Systems Programmer
Binghamton University

Jim Bohnsack
Cornell Univ.
[phone]

[Nick Laflamme]

Keep in mind, virtual disks, unlike temporary disks, are not subject to
fragmentation, AFAIK.

Nick

Mary--What you're seeing is fragmentation. TDISK is allocated in one
whole piece to a user. Think of PC disk allocation that you see when
you run a defrag. If you have 1 allocated cylinder out in the middle
of an otherwise empty 100 cylinder TDISK space, you cannot allocate a
99 cyl TDISK. You would get that by the first user getting 50
cylinders and the 2nd user getting 1 cylinder. When the first user
detaches the 50 cyl. disk, the 2nd user still has the 1 cyl in the
middle. You are left with 99 cylnders in total free but you can only
allocate a max of 50 cylinders and 49 cylinders.
Jim

Post by Mary Zervos
I have 500 cyl. of tdisk defined on one pack and 50 cyl. of tdisk
defined on another. One user used the 50 cyl. on the second pack
and about 24 users have used 215 cyl.(43%used) on the first pack.
Why is it that I can only allocate a 130 cyl. maximum tdisk on the
first pack but can then go ahead and allocate a 110 cyl. tdisk,
instead of being able to allocate *one* 240 cyl. tdisk?
Thanks for any help.
Mary Zervos
VM Systems Programmer
Binghamton University

Jim Bohnsack
Cornell Univ.
[phone]

[Schuh, Richard]

Mary,

If you have sufficient paging space, you might consider using V-disk instead of T-disk. As Nick pointed out, they are not subject to fragmentation and the limits for the individual users and the system as a whole can be set and adjusted dynamically. If you are not already paging constrained and you have the space, it might be a way out of the fragmentation problem. Look under HELP CP DEFINE (RELATED and select VDISK. You may also need to set limits using SET VDISK.

We have been using V-disk in place of T-disk for the past several years and have never had a problem with it.

-----Original Message-----
From: VM/ESA and z/VM Discussions [mailto:VMESA-***@LISTSERV.UARK.EDU] On Behalf Of Mary Zervos
Sent: Tuesday, January 24, 2006 6:43 AM
To: VMESA-***@LISTSERV.UARK.EDU
Subject: Re: Tdisk question

Thanks Jim for the insight. I had forgotten about fragmentation.

Mary

Mary--What you're seeing is fragmentation. TDISK is allocated in one
whole piece to a user. Think of PC disk allocation that you see when
you run a defrag. If you have 1 allocated cylinder out in the middle
of an otherwise empty 100 cylinder TDISK space, you cannot allocate a
99 cyl TDISK. You would get that by the first user getting 50
cylinders and the 2nd user getting 1 cylinder. When the first user
detaches the 50 cyl. disk, the 2nd user still has the 1 cyl in the
middle. You are left with 99 cylnders in total free but you can only
allocate a max of 50 cylinders and 49 cylinders.
Jim

Post by Mary Zervos
I have 500 cyl. of tdisk defined on one pack and 50 cyl. of tdisk
defined on another. One user used the 50 cyl. on the second pack
and about 24 users have used 215 cyl.(43%used) on the first pack.
Why is it that I can only allocate a 130 cyl. maximum tdisk on the
first pack but can then go ahead and allocate a 110 cyl. tdisk,
instead of being able to allocate *one* 240 cyl. tdisk?
Thanks for any help.
Mary Zervos
VM Systems Programmer
Binghamton University

Jim Bohnsack
Cornell Univ.
[phone]

[Mary Zervos]

Richard,

VDISK sounds like a new direction for our system too. Thanks.

Mary

Post by Schuh, Richard
Mary,
If you have sufficient paging space, you might consider using V-disk instead of T-disk. As Nick pointed out, they are not subject to fragmentation and the limits for the individual users and the system as a whole can be set and adjusted dynamically. If you are not already paging constrained and you have the space, it might be a way out of the fragmentation problem. Look under HELP CP DEFINE (RELATED and select VDISK. You may also need to set limits using SET VDISK.
We have been using V-disk in place of T-disk for the past several years and have never had a problem with it.
-----Original Message-----
Sent: Tuesday, January 24, 2006 6:43 AM
Subject: Re: Tdisk question
Thanks Jim for the insight. I had forgotten about fragmentation.
Mary

Mary--What you're seeing is fragmentation. TDISK is allocated in one
whole piece to a user. Think of PC disk allocation that you see when
you run a defrag. If you have 1 allocated cylinder out in the middle
of an otherwise empty 100 cylinder TDISK space, you cannot allocate a
99 cyl TDISK. You would get that by the first user getting 50
cylinders and the 2nd user getting 1 cylinder. When the first user
detaches the 50 cyl. disk, the 2nd user still has the 1 cyl in the
middle. You are left with 99 cylnders in total free but you can only
allocate a max of 50 cylinders and 49 cylinders.
Jim

Post by Mary Zervos
I have 500 cyl. of tdisk defined on one pack and 50 cyl. of tdisk
defined on another. One user used the 50 cyl. on the second pack
and about 24 users have used 215 cyl.(43%used) on the first pack.
Why is it that I can only allocate a 130 cyl. maximum tdisk on the
first pack but can then go ahead and allocate a 110 cyl. tdisk,
instead of being able to allocate *one* 240 cyl. tdisk?
Thanks for any help.
Mary Zervos
VM Systems Programmer
Binghamton University

Jim Bohnsack
Cornell Univ.
[phone]

[Alan Ackerman]

Many applications or userids go and get TDISK in their PROFILE EXEC, and then seldom or never
use it. Further, if they do have a problem, they increase the TDISK size, and never shrink it again.

Such users do lots of I/O to format the TDISK, and little else. Switching to VDISK will really help
here -- no formatting I/O at all. On the other hand if you have applications that write large
quantities of data to TDISK, perhaps as part of a sort, and then read it back in again, they may
swamp your paging system.

I don't know any easy way to tell the difference. One possibility is to limit VDISK per user, and go
for TDISK when they have used up that limit. That's easy to do if you have a common EXEC that
goes and gets TDISK, otherwise more difficult. (We issue USE TEMP100 to get 100 cylinders of
TDISK.)

We got an RVA, which did instant-format of TDISK, so we never used VDISK very much. We do use
it for Linux Swap, though.

I second (or third) the motion to consider VDISK. \
For years I was worried that it would swamp our limited real storage (1G).
But in real-life experience it has been FANTASTIC! Even with two 3390-3s
reserved for TDISK, we used to frequently run into fragmentation.
Formatting VDISK is instantaneous (regardless of size), and the I/O is
awesome (no moving parts!).
We've had a local TDISK EXEC here since almost day one. It just took a
small change, defaulting to VDISK (but allowed options to force it to be
allocated on with VDISK or TDISK). Happier (much) users, no more
fragmentation. Virtual life's great.
Mike Walter
Hewitt Associates
The opinions expressed herein are mine alone, not my employer's.