Position of the logical volume on the physical volume
Hello everyone,
I just read that while creating a logical volume(LV) we can choose the region of the physical volume (PV) in which the LV should be created.
When I say region I mean: outer edge - outer middle - center - inner middle and inner edge.
Can anyone help me understand the utility of that possibility ? How can we do that ( example please) ?
In smitty when you are creating a LV, you'll see:
You can change this to whatever you want. If you are using a SAN though, it probably won't make any difference since most modern SANs move data around as needed and spread it across multiple disks all unknown to the OS. If you are using internal physical disks, you will most likely see a difference - outer is faster than the inner because the head that reads the disk can read more on the outer edge because more disk is passed under it in a single disk rotation.
the center is the fastest on a disk - obviously this works only when you don't use raid 5 or SAN - you place it while you are creating the lv - either via smitty or with -a c option
The center is fastest? Outer tracks would have more sectors so as the disk spins, the head is able to read more sectors in a single disk rotation, right?
---------- Post updated at 10:13 AM ---------- Previous update was at 10:06 AM ----------
Here is a descent animation of the head seeking data on Western Digital's website. It is advertising their "IntelliSeek", but it shows how the head reads data. Western Digital
the outer edge of the disk has a faster rotational velocity than the slower inner edge and will whip around much more quickly on the outside than in the middle. However, because not all the data on the disk will be written to the outer edge as a result of the limitations of the physical placement of the data, the fastest seek times for the hard disk heads will be in the center area of the disk, where the head is most likely to pass on average.
I guess that is why UNIX gives the option of where you want the data because if you are worried about seek times, you'd want the data in the middle, if you were wanting raw speed to save a smaller number of large files, you'd be better off putting it on the outer edges... to each his own. Long live UNIX.
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IMHO the problem is placement within regions based on application activity.
The basic concept you want to follow: if the access is sequential in application terms the best I/O performance is achieved when the physical I/O access is sequential.
When application I/O is random there are many different things to consider - one of them being - is the I/O still physical, or is it VMM cached, or application cached.
In short, with modern systems (assuming more than one or two disks, i.e., separate volume groups for each application's data) there is no one best place.
Assuming some physical relationship the only advice that remains is that AIX LVM mirrored logical volumes with lots of write activity may benefit from being placed on the outer edge because the VGSA - Volume Group Status Area - is on the outer edge and gets updated anytime a mirrored LP is modified.
command you should look at: lslv - options: none, and with -l
lspv -p
This looks "bad" or less than ideal - 0% and in two regions - however, in physical terms it is a sequential as it gets. A simple command will improve the 0% to >50% (see below).
Here, I can see the logical volume is organized sequentially - starting in the center region, and proceeding into the "inner middle" region as one long sequence.
Now to get better stats I can do the following:
(change the preferred region to center from middle)
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Regards
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