Why does the # of blocks change for a file on a ZFS filesystem? Post: 302283178

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Operating Systems Solaris Why does the # of blocks change for a file on a ZFS filesystem? Post 302283178 by sqa777 on Monday 2nd of February 2009 06:29:15 PM

02-02-2009

Registered User

Why does the # of blocks change for a file on a ZFS filesystem?

I created a zpool and zfs filesystem in OpenSolaris. I made two NFS mount points:

> zpool history

History for 'raidpool':
2009-01-15.17:12:48 zpool create -f raidpool raidz1 c4t1d0 c4t2d0 c4t3d0 c4t4d0 c4t5d0
2009-01-15.17:15:54 zfs create -o mountpoint=/vol01 -o sharenfs=on -o canmount=on raidpool/vol01
2009-01-15.17:20:13 zfs create -o mountpoint=/vol02 -o sharenfs=on -o canmount=on -o compression=lzjb raidpool/vol02

I did not make the mountpoints (vol01 and vol02) into volumes. I know you can set default blocksizes when you create volumes but you cannot make them exportable NFS exports.

I am assuming that vol01 and vol02 are variable blocksizes because I did not explicitly specify a blocksize. Thus, my assumption is that ZFS would use a blocksize that is the the smallest power of 2 and the smallest blocksize is 512 bytes.

I use the stat command to check the filesize, the blocksize, and the # of blocks.

I created a file that is exactly 512 bytes in size on /vol01 (the one without the LZ compression)

I do the following stat command:
stat --printf "%n %b %B %s %o\n" *

The %b is the number of blocks used.

The number of blocks changes after a few minutes after the file is created:

# stat --printf "%n %b %B %s %o\n" *
file.0 3 512 3 4096
file.512 1 512 512 4096
# stat --printf "%n %b %B %s %o\n" *
file.0 3 512 3 4096
file.512 1 512 512 4096
# stat --printf "%n %b %B %s %o\n" *
file.0 3 512 3 4096
file.512 3 512 512 4096

Why does the # of blocks change after a few minutes? And why are we using 3 blocks when the file is only 512 bytes in size (in other words, only 1 block is needed)???

sqa777

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LEARN ABOUT DEBIAN

amzfs-sendrecv

AMZFS-SENDRECV(8)					  System Administration Commands					 AMZFS-SENDRECV(8)

NAME

       amzfs-sendrecv - Amanda script to create zfs sendrecv

DESCRIPTION

       amzfs-sendrecv is an Amanda application implementing the Application API. It should not be run by users directly. It create a zfs snapshot
       of the filesystem and backup the snapshot with 'zfs send'. Snapshot are kept after the backup is done, this increase the disk space use on
       the client but it is neccesary to be able do to incremental backup. If you want only full backup, you can disable this feature by setting
       the KEEP-SNAPSHOT property to 'NO'. Only the restoration of the complete backup is allowed, it is impossible to restore a single file.

       The application is run as the amanda user, it must have many zfs priviledge:

	   zfs allow -ldu AMANDA_USER mount,create,rename,snapshot,destroy,send,receive FILESYSTEM

       Some system doesn't have "zfs allow", but you can give the Amanda backup user the rights to manipulate ZFS filesystems by using the
       following command:

       usermod -P "ZFS File System Management,ZFS Storage Management" AMANDA_USER
       This will require that your run zfs under pfexec, set the PFEXEC property to YES.

       The format of the diskdevice in the disklist (DLE) must be one of:

       Desciption	       Example
       ----------	       -------
       Mountpoint	       /data
       ZFS pool name	       datapool
       ZFS filesystem	       datapool/database
       ZFS logical volume      datapool/dbvol

       The filesystem doesn't need to be mounted.

PROPERTIES

       This section lists the properties that control amzfs-sendrecv's functionality. See amanda-applications(7) for information on the
       Application API, application configuration.

       DF-PATH

	   Path to the 'df' binary, search in $PATH by default.

       KEEP-SNAPSHOT

	   If "YES" (the default), snapshot are kept after the backup, if set to "NO" then snapshot are no kept and incremental backup will fail.

       ZFS-PATH

	   Path to the 'zfs' binary, search in $PATH by default.

       PFEXEC-PATH

	   Path to the 'pfexec' binary, search in $PATH by default.

       PFEXEC

	   If "NO" (the default), pfexec is not used, if set to "YES" then pfexec is used.

EXAMPLE

       In this example, a dumptype is defined to use amzfs-sendrecv application to backup a zfs filesystem.
	define application-tool amzfs_sendrecv {
	  comment "amzfs-sendrecv"
	  plugin "amzfs-sendrecv"
	  #property "DF-PATH"  "/usr/sbin/df"
	  #property "KEEP-SNAPSHOT" "YES"
	  #property "ZFS-PATH" "/usr/sbin/zfs"
	  #property "PFEXEC-PATH" "/usr/sbin/pfexec"
	  #property "PFEXEC" "NO"
	}

	define dumptype user-zfs-sendrecv {
	  program "APPLICATAION"
	  application "amzfs_sendrecv"
	}

SEE ALSO

       amanda(8), amanda.conf(5), amanda-client.conf(5), amanda-applications(7)

       The Amanda Wiki: : http://wiki.zmanda.com/

AUTHOR

       Jean-Louis Martineau <martineau@zmanda.com>
	   Zmanda, Inc. (http://www.zmanda.com)

Amanda 3.3.1							    02/21/2012							 AMZFS-SENDRECV(8)