Hi.
This is information of filesystem /movelv_test. It only used 42% capacity / 20 physical partions ( PP ) each PV. As I understand, it means that only 8 PP on each PV contain data, 12 PP remaining on each PV are assigned to logical volume fslv01 but have no data. Is this true ???
If yes, how to know exactly which PP contains data.
Thank for read.
Last edited by bobochacha29; 12-09-2014 at 02:30 AM..
This is information of filesystem /movelv_test. It only used 42% capacity / 20 physical partions ( PP ) each PV.
Not quite. The filesystem "/movelv_test" is residing on a certain Logical Volume, "fslv01". This LV in turn is made from 20 Logical Partitions. Because the LV is mirrored each LP is comprised from 2 Physical Partitions. Would the LV not be mirrored it would still be made from 20 LPs but this time each LP would be made from only one PP.
You need to understand that there are several abstraction layers and you cannot change between them at will.
Quote:
Originally Posted by bobochacha29
As I understand, it means that only 8 PP on each PV contain data, 12 PP remaining on each PV are assigned to logical volume fslv01 but have no data. Is this true ???
No. You seem to think that data fills the raw disk space like water fills a bucket - from the bottom up, but this is not the case. Filesystems are made for random read/write access and that means that filling them contiguously would be a performance nightmare. Suppose you have two files, A and B and they are put adjacently on the disk. When you edit file A now and add a single character you would have no space to write that, so you need to rewrite the whole file. If there is a way to fragment that file and put the additional character elsewhere you could leave most of the file in place.
The relationship between disk space and filesystem space is more like the price you pay for something: suppose you purchase a house for some money. You can point to the whole money and say it bought you the whole house, but the question which bills bought me the western wall just makes no sense.
Of course you can investigate where exactly on which part of the disk a certain file is. But be aware that when you use the FS this can change over time and it is by no means fixed.
I don't really understand what you means. As you said above, data is written randomly to the disk and these lp/pp listed above have no relationship with the filesytem space used/free. The data may reside anywhere on these 40 pps and there's no way to know that. Is that what you means?
As you said above, data is written randomly to the disk and these lp/pp listed above have no relationship with the filesytem space used/free.
Yes and no. data is not written entirely randomly. There is a system behind it, but it is complex. You seem to have meant that a filesystem which is, say, 50% full and consists of LPs1-10 will have the LPs1-5 written full and LPs6-10 empty. This is (typically) not the case. Typically (depending on what the content of your filesystem is - many small files or a few large files or some mix of both, how often the data on disk changes and some other specifics) your filesystems space will be scattered and every LP will be nearly 50% full and nearly 50% empty.
Furthermore, if a FS resides on a LV which consists of 10 LPs these are ordered 1-10. But that does not have to mean that LP3 is "behind" LP2 on disk. It could well be that LP1 is at PP200, LP2 is at PP100 and LP3 is at PP150 (with maybe other LVs taking the place in between). For the filesystem the space would still appear to be linear, first the space coming from LP1, then from LP2, etc.. The FS will be unaware of the fact that to go from LP1 to LP2 this means to skip many PPs on disk.
Quote:
Originally Posted by bobochacha29
The data may reside anywhere on these 40 pps and there's no way to know that.
Yes, it is possible to find out, but no, there is no 1:1-relation you could use. Say, you have some file. If you want to know where the first byte of the file "/path/to/some/file" is located on the disk you can find out on which place the first (filesystem) block of your file is. Then, using LV methods, find out on which LP this part of the filesystem really is. After this, you use the LP-PP-relation you already have posted to find out which PP is used to represent this LP and finally you can use VG-methods to locate the PP on some specific place on some specific disk and use low-level methods ("dd" and the like) to really find that part of the file you started with.
But, having found that out for byte X doesn't mean that you could read the next byte on disk and expect to find the content of byte X+1 in the file. This may be so or not, but to know that you will need to exercise the whole procedure again.
Are you wanting to shrink the filesystem, replace a disk or something? I'm wondering why you are so interested.
To shrink a filesystem on AIX is not simple task like it can by on some other operating systems, for instance RHEL 6 has an option to re-size up or down the filesystem and logical volume all in one command, although it requires it to unmount as part of the process.
On AIX you would need to either:-
Build a new filesystem of the required size, move the data and then remount the new filesystem to the correct location (making the change permanent for next boot etc.)
Backup the data, destroy and re-create the filesystem and then restore the data.
If you are looking to free up or replace a disk, then migratepv is your friend here. It can take a while depending how big your filesystem is, but it will (one at a time) make a copies the LPs to the target drive then remove the old one. Do not interrupt the process as this can leave you with inconsistencies in the volume group and logical volume information.
Code:
migratepv -l logical_volumesource_disktarget_disk
An alternate would be to use mirrors to achieve this if placement is critical. You can use mklvcopy to add a third copy to the new disk (and synchronise them) and then use rmlvcopy specifying to remove the one from the old disk. When making the 3rd copy, you can specify a map-file to force it to use the PPs on whichever disk in whichever order you want if that is important.
Whichever technique you use, you can empty a disk and remove it from the volume group for replacement or use elsewhere.
If I've missed the point, then let us know what you really need to know and we will try to help.
To shrink a filesystem on AIX is not simple task like it can by on some other operating systems, for instance RHEL 6 has an option to re-size up or down the filesystem and logical volume all in one command, although it requires it to unmount as part of the process.
Actually this is correct only historically. Since IIRC version 5.3 it is possible to shrink a filesystem. Initially it was possible only if the end of the underlying LV (from where the shrinking took place) was not already used by files but since then AIX does a "reorgfs" automatically as it seems. The only restricition (obviously) is that the resulting size must be big enough to hold all the existing data.
To shrink a FS (and the underlying LV as well) do, analogous to a growth:
Code:
chfs -a size=-<some size> /path/to/mountpoint
I hope this helps.
bakunin
These 2 Users Gave Thanks to bakunin For This Post:
Are you wanting to shrink the filesystem, replace a disk or something? I'm wondering why you are so interested.
To shrink a filesystem on AIX is not simple task like it can by on some other operating systems, for instance RHEL 6 has an option to re-size up or down the filesystem and logical volume all in one command, although it requires it to unmount as part of the process.
On AIX you would need to either:-
Build a new filesystem of the required size, move the data and then remount the new filesystem to the correct location (making the change permanent for next boot etc.)
Backup the data, destroy and re-create the filesystem and then restore the data.
If you are looking to free up or replace a disk, then migratepv is your friend here. It can take a while depending how big your filesystem is, but it will (one at a time) make a copies the LPs to the target drive then remove the old one. Do not interrupt the process as this can leave you with inconsistencies in the volume group and logical volume information.
Code:
migratepv -l logical_volumesource_disktarget_disk
An alternate would be to use mirrors to achieve this if placement is critical. You can use mklvcopy to add a third copy to the new disk (and synchronise them) and then use rmlvcopy specifying to remove the one from the old disk. When making the 3rd copy, you can specify a map-file to force it to use the PPs on whichever disk in whichever order you want if that is important.
Whichever technique you use, you can empty a disk and remove it from the volume group for replacement or use elsewhere.
If I've missed the point, then let us know what you really need to know and we will try to help.
Regards,
Robin
Hehe, how do you know that. It's true that I'm having problem with filesystem, but it's a little different with what you think. In fact, I want to split the data of a filesystem on multiple hard disks equally.
It's quite a waste of lines listing all the lp/pp of the real filesystem I'm handling, so allow me to use that /movelv_test as an example.
( Don't be surpised why it looks like that. At first, the vg of this fs have only 2 pv, hdisk1 and hdisk2. The data grew up day by day and when there was no free pp on hdisk1 and hdisk2, we added two more pv: hdisk3 and hdisk4 to the vg and then increase the /movelv_test size . Now this fs 's Used% is ~ 55%, 180GB assigned to the fs/ 100GB data used )
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 ).
The result of the "topas" command and the lp/pp listed above made me think that data fills the raw disk space like water fills a bucket - as bakunin said, but now I know it's not true.
Now, what I want to do is to balance the disk operation of these 4 pv ( just only at this time,temporarily don't care about data increasing ) and I intend to do like this:
_ Decrease the size of the fs so that the fs's Used% ~ 99% ( to ensure that most of the pp are filled with data and don't care whether the pp contains data or not ), in this case - decrease size of /movelv_test to 101 GB
_ Used "migratelp" command to arrange each pp of the fslv01 like this
_ Increase the size of the fs back ( in this case is 180 GB )
And I have two questions
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 ???
2. Does this solution work ? Are there any solutions better?
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12-09-2014
