How to know exactly which physical partion contains data? Post: 302928469

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Operating Systems AIX How to know exactly which physical partion contains data? Post 302928469 by bakunin on Friday 12th of December 2014 02:49:12 AM

12-12-2014

Registered User

Quote:

Originally Posted by bobochacha29

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.

I hope this helps.

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LEARN ABOUT REDHAT

xfs_freeze

xfs_freeze(8)						      System Manager's Manual						     xfs_freeze(8)

NAME

       xfs_freeze - suspend access to an XFS filesystem

SYNOPSIS

       xfs_freeze -f | -u mount-point

DESCRIPTION

       xfs_freeze suspends and resumes access to an XFS filesystem (see xfs(5)).

       xfs_freeze  halts  new access to the filesystem and creates a stable image on disk.  xfs_freeze is intended to be used with volume managers
       and hardware RAID devices that support the creation of snapshots.

       The mount-point argument is the pathname of the directory where the filesystem is mounted.  The filesystem must be  mounted  to	be  frozen
       (see mount(8)).

       The  -f flag requests the specified XFS filesystem to be frozen from new modifications.	When this is selected, all ongoing transactions in
       the filesystem are allowed to complete, new write system calls are halted, other calls which modify the	filesystem  are  halted,  and  all
       dirty data, metadata, and log information are written to disk.  Any process attempting to write to the frozen filesystem will block waiting
       for the filesystem to be unfrozen.

       Note that even after freezing, the on-disk filesystem can contain information on files that are still in the process of	unlinking.   These
       files will not be unlinked until the filesystem is unfrozen or a clean mount of the snapshot is complete.

       The  -u	flag  is used to un-freeze the filesystem and allow operations to continue.  Any filesystem modifications that were blocked by the
       freeze are unblocked and allowed to complete.

       One of -f or -u must be supplied to xfs_freeze.

NOTES

       A copy of a frozen XFS filesystem will usually have the same universally unique identifier (UUID) as the original, and  thus  may  be  pre-
       vented from being mounted.  The XFS nouuid mount option can be used to circumvent this issue.

       In  Linux  kernel version 2.6.29, the interface which XFS uses to freeze and unfreeze was elevated to the VFS, so that this tool can now be
       used on many other Linux filesystems.

SEE ALSO

       xfs(5), lvm(8), mount(8).

																     xfs_freeze(8)