Concept of ZFS transaction semantics Post: 302561103

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Operating Systems Solaris Concept of ZFS transaction semantics Post 302561103 by h@foorsa.biz on Monday 3rd of October 2011 06:59:00 AM

10-03-2011

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Transactional Semantics

ZFS is a transactional file system, which means that the file system state is always consistent on disk. Traditional file systems overwrite data in place, which means that if the machine loses power, for example, between the time a data block is allocated and when it is linked into a directory, the file system will be left in an inconsistent state. Historically, this problem was solved through the use of the fsck command. This command was responsible for going through and verifying file system state, making an attempt to repair any inconsistencies in the process. This problem caused great pain to administrators and was never guaranteed to fix all possible problems. More recently, file systems have introduced the concept of journaling. The journaling process records action in a separate journal, which can then be replayed safely if a system crash occurs. This process introduces unnecessary overhead, because the data needs to be written twice, and often results in a new set of problems, such as when the journal can't be replayed properly.

With a transactional file system, data is managed using copy on write semantics. Data is never overwritten, and any sequence of operations is either entirely committed or entirely ignored. This mechanism means that the file system can never be corrupted through accidental loss of power or a system crash. So, no need for a fsck equivalent exists. While the most recently written pieces of data might be lost, the file system itself will always be consistent. In addition, synchronous data (written using the O_DSYNC flag) is always guaranteed to be written before returning, so it is never lost.

In short words ,ZFS Transactional Semantics is intend to keep your data intact , safe and consistent state.

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LEARN ABOUT FREEBSD

zfsboot

ZFSBOOT(8)						    BSD System Manager's Manual 						ZFSBOOT(8)

NAME

     zfsboot -- bootcode for ZFS on BIOS-based computers

DESCRIPTION

     zfsboot is used on BIOS-based computers to boot from a filesystem in a ZFS pool.  zfsboot is installed in two parts on a disk or a partition
     used by a ZFS pool.  The first part, a single-sector starter boot block, is installed at the beginning of the disk or partition.  The second
     part, a main boot block, is installed at a special offset within the disk or partition.  Both areas are reserved by the ZFS on-disk specifi-
     cation for boot use.  If zfsboot is installed in a partition, then that partition should be made bootable using appropriate configuration and
     boot blocks described in boot(8).

BOOTING

     The zfsboot boot process is very similar to that of gptzfsboot(8).  One significant difference is that zfsboot does not currently support the
     GPT partitioning scheme.  Thus only whole disks and MBR partitions, traditionally referred to as slices, are probed for ZFS disk labels.  See
     the BUGS section in gptzfsboot(8) for some limitations of the MBR scheme support.

USAGE

     zfsboot supports all the same prompt and configuration file arguments as gptzfsboot(8).

FILES

     /boot/zfsboot  boot code binary
     /boot.config   parameters for the boot block (optional)
     /boot/config   alternative parameters for the boot block (optional)

EXAMPLES

     zfsboot is typically installed using dd(1).  To install zfsboot on the ada0 drive:

	   dd if=/boot/zfsboot of=/dev/ada0 count=1
	   dd if=/boot/zfsboot of=/dev/ada0 iseek=1 oseek=1024

     If the drive is currently in use, the GEOM safety will prevent writes and must be disabled before running the above commands:

	   sysctl kern.geom.debugflags=0x10

     zfsboot can also be installed in an MBR slice:

	   gpart create -s mbr ada0
	   gpart add -t freebsd ada0
	   gpart create -s BSD ada0s1
	   gpart bootcode -b /boot/boot0 ada0
	   gpart set -a active -i 1 ada0
	   dd if=/boot/zfsboot of=/dev/ada0s1 count=1
	   dd if=/boot/zfsboot of=/dev/ada0s1 iseek=1 oseek=1024

     Note that commands to create and populate a pool are not shown in the example above.

SEE ALSO

     dd(1), boot.config(5), boot(8), gptzfsboot(8), loader(8), zfsloader(8), zpool(8)

HISTORY

     zfsboot appeared in FreeBSD 7.3.

AUTHORS

     This manual page was written by Andriy Gapon <avg@FreeBSD.org>.

BUGS

     Installing zfsboot with dd(1) is a hack.  ZFS needs a command to properly install zfsboot onto a ZFS-controlled disk or partition.

BSD
								September 15, 2014							       BSD