1. Is it OK to use the command " migratelp" to do something like this. The data availability, the synchronization between the 1st mirror and the 2nd mirror ... is OK ? Are there any errors of the filesystem, errors of data, ... after using this command? If 1 of 4 hard disk fails, the data is still OK ???
It is perfectly OK and in fact the command was made for exactly this purpose. Still, i think you do not need it, see below.
Quote:
Originally Posted by bobochacha29
In fact, I want to split the data of a filesystem on multiple hard disks equally.
[...]
The problem is when I checked the disk operation ( by command "topas" ), I found that the operation of hdisk1 and hdisk2 is very high, ~90%-100% while the operation of hdisk3 and hdisk4 is just ~ 10% ( most of the disk operation is writing - the application writes logs to the fs's mountpoint ).
This - the splitting you intend - makes sense only in a specific kind of situation, so please describe your hdisk-devices a bit better. What are they (single SCSI-disks, RAID-sets, LUNs from SAN, ...) and how do you access them?
First a little theory, so that ou can understand the output better:
Here you see several of the "layers" i talked about in a previous post at work: The LV consists of LPs (leftmost column) numbered 0001, 0002, 0003, ... and the space in this LV is continuous. That means that when byte nr. X is the last byte in in LP 0001 then byte nr X+1 is the first byte in LP 0002. Now, the LP 0001 consists in fact of two PPs, which hold identical copies; PP0175 on hdisk1 and PP 0111 on hdisk2. Similarily for all the other LPs.
The first question is if you really need the LV-copies. writing in parallel is a tad slower than writing on a single LV (without copies) and it might help with the performance if you do away with the mirroring. You will have to decide if the loss of security outweighs the gain on performance or if it is the other way round.
Second, you can place the LPs also in this way (schematically):
The LVM of AIX has a special provision for doing that, called (somewhat counterintuitively) "Inter-Policy", look here:
Code:
# lslv -l mylv
mylv:/some/where
PV COPIES IN BAND DISTRIBUTION
hdiskpower2 1670:000:000 24% 270:410:409:409:172
# lslv -L ap33p1lv
LOGICAL VOLUME: mylv VOLUME GROUP: myvg
LV IDENTIFIER: 00534b7a00004c000000011cd5e0067e.1 PERMISSION: read/write
VG STATE: active/complete LV STATE: opened/syncd
TYPE: jfs2 WRITE VERIFY: off
MAX LPs: 4096 PP SIZE: 512 megabyte(s)
COPIES: 1 SCHED POLICY: parallel
LPs: 1670 PPs: 1670
STALE PPs: 0 BB POLICY: relocatable
INTER-POLICY: minimum RELOCATABLE: yes
INTRA-POLICY: middle UPPER BOUND: 32
MOUNT POINT: /some/where LABEL: /some/where
MIRROR WRITE CONSISTENCY: on/ACTIVE
EACH LP COPY ON A SEPARATE PV ?: yes
Serialize IO ?: NO
INFINITE RETRY: no
At "minimum" the LVM places the LPs on the PPs in a way so that the minimum possible hdisks are involved. At "maximum" it will try to spread it over as many hdisks as possible, thus arriving at a placement similar to what i have sketched out above.
Note, btw, that the smaller the PP size you use (this is a property of the VG so you might have to create it anew and start over from scratch) the better it is for the effect to take place. You are ultimately trying to use the internal cache of the hdisks for maximum effect and the PPs should be small enough to fit into this cache. 512MB, like in my example, would be too big for that.
If you really have to create the VG anew it might also be a good idea to create a RAID set (faster than single disks but slower than a stripe set) or even a stripe set (faster than the RAID but lacking the security of the RAID). See this little tutorial for details about RAIDs, striping, etc..
You can even more fine-tune the creation process of the LV by using a so-called "map file" and using the "-m" switch (see the man page of "mklv" for details). Basically you can explicitly state which PP(s) should represent any given LP of the LV that way.
Further, please tell us which kind of data the FS holds. You already said "mostly writing log files", but a little more detail would help: many small files, a few very large files, do the files change often or are they mostly appended? How often are files deleted and recreated (like in log rotation)? How many processes write (typically) concurrently to the FS. It might be that you can gain a lot with different OS tuning parameters without even having to change the disk layout.
At last, about "migratepv": if you really need it (which is, as shown above, not sure IMHO) you will want to remove any mirror copy from the LV prior to migrating its PPs around. It is simply only half the work because every PP has to moved separately. Unmirror the LV you want to migrate, do the migration and when that is done create a new mirror. You can use a map file (see above, the "-m" switch) for this too.
Can I also raise a concern about your plan to write LPs from both copies spread over the same disks. What would happen if you lost hdisk3, for instance. You might end up with two copies both with some missing LPs and a lot of hard work to extricate yourself.
I've not found a mention of reducing FS size in the manual page for chfs on AIX 6. The details about the size= part are below:-
Quote:
-a size=NewSize
Specifies the size of the Journaled File System. The size can be specified in units of 512-byte blocks, megabytes or
gigabytes. If Value has the M suffix, it is interpreted to be in megabytes. If Value has a G suffix, it is interpreted
to be in gigabytes. If Value begins with a +, it is interpreted as a request to increase the file system size by the
specified amount. If the specified size is not evenly divisible by the physical partition size, it is rounded up to the
closest number that is evenly divisible.
The process to reduce the FS does work though
Code:
# lslv robin_test_lv
LOGICAL VOLUME: robin_test_lv VOLUME GROUP: rebsvg
LV IDENTIFIER: 00cfe9f500004c000000012bcecff0de.18 PERMISSION: read/write
VG STATE: active/complete LV STATE: opened/syncd
TYPE: jfs2 WRITE VERIFY: off
MAX LPs: 512 PP SIZE: 32 megabyte(s)
COPIES: 2 SCHED POLICY: parallel
LPs: 32 PPs: 64
STALE PPs: 0 BB POLICY: relocatable
INTER-POLICY: minimum RELOCATABLE: yes
INTRA-POLICY: edge UPPER BOUND: 32
MOUNT POINT: /robin_test_fs LABEL: /robin_test_fs
MIRROR WRITE CONSISTENCY: on/ACTIVE
EACH LP COPY ON A SEPARATE PV ?: yes
Serialize IO ?: NO
INFINITE RETRY: no
# chfs -a size=-32M /robin_test_fs
Filesystem size changed to 2031616
# lslv robin_test_lv
LOGICAL VOLUME: robin_test_lv VOLUME GROUP: rebsvg
LV IDENTIFIER: 00cfe9f500004c000000012bcecff0de.18 PERMISSION: read/write
VG STATE: active/complete LV STATE: opened/syncd
TYPE: jfs2 WRITE VERIFY: off
MAX LPs: 512 PP SIZE: 32 megabyte(s)
COPIES: 2 SCHED POLICY: parallel
LPs: 31 PPs: 62
STALE PPs: 0 BB POLICY: relocatable
INTER-POLICY: minimum RELOCATABLE: yes
INTRA-POLICY: edge UPPER BOUND: 32
MOUNT POINT: /robin_test_fs LABEL: /robin_test_fs
MIRROR WRITE CONSISTENCY: on/ACTIVE
EACH LP COPY ON A SEPARATE PV ?: yes
Serialize IO ?: NO
INFINITE RETRY: no
How delighted am I?
Given that you can add a PP by using chfs -a size=+1 I tried chfs -a size=-1 /test_robin_fs and got en error, which I eventually deciphered as the reduction appears to need to be given in multiples of the PP size.
Sorry for replying so late. It took me a few days to check the data I/O to do the migrate, and a few days more to check the result after the migration.
Quote:
This - the splitting you intend - makes sense only in a specific kind of situation, so please describe your hdisk-devices a bit better. What are they (single SCSI-disks, RAID-sets, LUNs from SAN, ...) and how do you access them?
They are SCSI Disk Drive.
Quote:
Further, please tell us which kind of data the FS holds. You already said "mostly writing log files", but a little more detail would help: many small files, a few very large files, do the files change often or are they mostly appended? How often are files deleted and recreated (like in log rotation)? How many processes write (typically) concurrently to the FS. It might be that you can gain a lot with different OS tuning parameters without even having to change the disk layout.
Yes, many small files and these small files are compressed to a tar file after each day. Depending on which kinds of log, these tar files are deleted after 1 week, 1 month or 1 years ...
This i's what I've done for the last few days. I used "lvmstat" to collect the information of I/O stat of each lp/pp of fslv00.
You can see that, however the "KB-Writ" of hdisk4 and hdisk5 is higher than these of hdisk0 and hdisk3, the "Busy%" of hdisk0 and hdisk3 is higher than these of hdisk4 and hdisk5.
It's so complicated.
Last edited by rbatte1; 12-29-2014 at 06:05 AM..
Reason: Change CODE tags to QUOTE tags for text quoted from bakunin. - RBATTE1 corrected some spelling.
Of course it is. If systems administration would be simple we wouldn't be the heroes of the whole IT business, would we? So welcome to the job with the biggest demands and the greatest rewards our industry has to offer.
Quote:
Originally Posted by bobochacha29
,
They are SCSI Disk Drive.
Yes, many small files and these small files are compressed to a tar file after each day. Depending on which kinds of log, these tar files are deleted after 1 week, 1 month or 1 years ...
Well, this is something we can build on. I can help you better when i return to the office (and the documentation: there is a lot of detail i do not know from the top of my head).
Quote:
Originally Posted by bobochacha29
You can see that, however the "KB-Writ" of hdisk4 and hdisk5 is higher than these of hdisk0 and hdisk3, the "Busy%" of hdisk0 and hdisk3 is higher than these of hdisk4 and hdisk5.
A little general information about the disk statistics and what they mean:
Every disk has a "command queue": read- and write-requests are buffered and then worked on one after the other. If the queue is full the disk will not accept more commands until some room in the queue is free again. Keep this in mind for a moment.
The OS now asks every disk (in this regard, "disk" means "everything with a "hdisk"-device - it does not have to be a physical disk but also a LUN, a RAID-set, ....) in turn if this queue has length 0 at the moment or not, which the disk answers with "yes" (= length 0) or "no" (any length different then 0). From many of these answers the OS compiles a percentage which is shown as "disk busy %".
This means that "disk busy" is not as important as you think and that it has no meaning by itself. If a queue has "not length 0" it can have length 1 or length 15. The value is interesting because you get a measure of how many accesses a disk experiences. But you cannot measure the throughput of a device from that value alone. Disk operations come in different sizes, varying from 512 bytes (one disk block) up to several GB. Which of these is outstanding the busy% will not tell you, just that there is one outstanding at all.
You might consider balancing for "I/O" rather than for "busy%", but you might even do something different. I suggest you read my little introduction to performance tuning with emphasis to I/O-tuning in the meantime. I will come back to this thread once i am back in office (next Monday).
I hope this helps.
bakunin
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