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:
Quote:
Originally Posted by bobochacha29
This is how it looks like
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:
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.
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