DRBD-Cookbook. Joerg Christian Seubert
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Название: DRBD-Cookbook
Автор: Joerg Christian Seubert
Издательство: Bookwire
Жанр: Математика
isbn: 9783754950326
isbn:
3.2.2 ext3 / ext4
The ext3 and ext4 file systems have evolved from the ext2 with the addition of a so-called journal and the ability to change the size of the file system while the file system is in use.
In a journaling file system, all changes are recorded in a special memory area called journal before the actual write to the selected block takes place. This makes it easier to reconstruct the writes if, for example, the system crashes or the power goes out during the write operation.
Another point of improvement of ext3 resp. ext4 over ext2 was the increase in file system partitions from 16 TB to 32 TB for ext3 and 1 EB (= exabyte) for ext4. Such device sizes could not have been imagined when the UFS was developed.
In addition, there are the extensions regarding the number of files and directories as well as the size of the individual files, which was still limited to 2 TB for ext2, could be between 16 GB and 2 TB for ext3, and is finally only limited by the size of the disk partition for ext4.
3.2.3 xfs
The file system xfs, originally developed by Silicon Graphics (SGI) exclusively for the in-house UNIX system "IRIX", is one of the oldest file systems. But just because something is getting on in years doesn’t mean it has to be "bad". It sets standards with maximum values of 16 EB per file system, a maximum number of 263 files and a size per file of 8 EB.
It also has significant advantages over ext3 and BtrFS, especially in terms of speed.
Some time ago, I had a case where about 100 GB needed to be copied from one host to another. The source filesystem was a BtrFS and the copy ran - to save LAN resources - over a TAR that was compressed on the source machine, pushed through an SSH tunnel and decompressed again on the target machine.
This work took a little over an hour - probably because the file system had many subdirectories.
After the work on the source file system was finished, among other things it was enlarged to 200 GB, I spontaneously decided to use xfs as the new file system.
The recovery time was 20 minutes!
Needless to say, I have been a self-confessed fan of this file system since that time, especially because the throughput has been confirmed in normal operation.
3.2.4 BtrFS
The BtrFS - spelled out B-Tree file system and not "Better FS" or even "Butter FS" - follows a completely different approach than the file systems available in the Linux environment so far. It is based partly on the considerations of the ZFS, which was developed about seven years before also by Sun Microsystems (in the meantime merged into ORACLE). It has built-in RAID, volume management, checksum-based protection against data transfer errors, and uses copy-on-write. copy-on-write is a method where a copy is not real until it is changed by one of the parties. As long as all parties involved have not changed their copy, it is sufficient to save the original once - in the respective file system. The integrated RAID system distinguishes between occupied and free data blocks, so when reconstructing a failed RAID volume, only the occupied disk space needs to be mirrored, which saves an enormous amount of time. In addition, this RAID works with larger data blocks than is the case with classic RAID methods. In a RAID1, there is no mirroring of all data blocks of a data carrier - regardless of whether they are occupied or not - but only the occupied blocks are distributed to all available data carriers. In this way, a RAID1 can be formed from an odd number of disks with different capacities without losing storage space. The B-tree structure - after which the file system is named - comes from the central concept of xfs. BtrFS is now used by SuSE as the file system of the future, while RedHat announced in August 2017 that it would discontinue long-term support for BtrFS in RHEL. Whereas so far it has remained with this announcement and for example fedora1 uses BtrFS quite automatically unless something else is chosen during installation. In addition to the experience described above, the following considerations should be made with regard to using BtrFS on production servers in conjunction with DRBD:
1 Most Linux servers purchased as 19-inch devices are equipped with a hardware RAID controller. This means that the RAID functionality of BtrFS is not needed here, since the hard disks connected to the RAID array have the same capacity anyway, otherwise the hardware RAID controller will not work properly or the storage space cannot be used.
2 The copy-on-write functionality explained above adds an additional virtualization layer that is already done by distributing the data to multiple cluster nodes with DRBD. However, a BtrFS array across multiple cluster nodes is not supported.
3 In certain cases, using BtrFS on a DRBD device makes perfect sense if you don’t want to miss out on the multiple functionalities of BtrFS.
An example is the possibility to create a snapshot of the file system. You should, however, be careful before automatically creating such snapshots, as this quickly consumes disk space that you might need for other purposes.
3.2.5 OCFS2
The Oracle Cluster File System 2 is a file system that enables simultaneous access from several cluster nodes to a disk device in a cluster array (concurrent access), developed by Oracle for open source clusters. The coordination works via the Distributed Lock Manager.
The required packages are included as of SLES 11 SP3.
3.2.6 Conclusion
You should also think very carefully about which file system you want to use during the basic considerations for setting up a cluster. If you have made the wrong decision, it is only possible to change the file system with increased effort. In this book I show a "recipe" (cf. text number 8 - without increasing the LVM volume) how you can help yourself here. However, you must be aware that this can only be achieved with a downtime of the cluster array. If you are comparing the file systems shown here and do not need the special features that OCFS2 or BtrFS bring, xfs is the method of choice.
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