<emphasis>PERSPECTIVE</emphasis> Btrfs combines LVM and RAID
<emphasis>观点</emphasis> Btrfs 融合 LVM 和 RAID
While LVM and RAID are two distinct kernel subsystems that come between the disk block devices and their filesystems, <emphasis>btrfs</emphasis> is a new filesystem, initially developed at Oracle, that purports to combine the featuresets of LVM and RAID and much more. It is mostly functional, and although it is still tagged “experimental” because its development is incomplete (some features aren't implemented yet), it has already seen some use in production environments. <ulink type="block" url="" />
Among the noteworthy features are the ability to take a snapshot of a filesystem tree at any point in time. This snapshot copy doesn't initially use any disk space, the data only being duplicated when one of the copies is modified. The filesystem also handles transparent compression of files, and checksums ensure the integrity of all stored data.
In both the RAID and LVM cases, the kernel provides a block device file, similar to the ones corresponding to a hard disk drive or a partition. When an application, or another part of the kernel, requires access to a block of such a device, the appropriate subsystem routes the block to the relevant physical layer. Depending on the configuration, this block can be stored on one or several physical disks, and its physical location may not be directly correlated to the location of the block in the logical device.
Software RAID
RAID means <emphasis>Redundant Array of Independent Disks</emphasis>. The goal of this system is to prevent data loss in case of hard disk failure. The general principle is quite simple: data are stored on several physical disks instead of only one, with a configurable level of redundancy. Depending on this amount of redundancy, and even in the event of an unexpected disk failure, data can be losslessly reconstructed from the remaining disks.
<emphasis>CULTURE</emphasis> <foreignphrase>Independent</foreignphrase> or <foreignphrase>inexpensive</foreignphrase>?
The I in RAID initially stood for <emphasis>inexpensive</emphasis>, because RAID allowed a drastic increase in data safety without requiring investing in expensive high-end disks. Probably due to image concerns, however, it is now more customarily considered to stand for <emphasis>independent</emphasis>, which doesn't have the unsavory flavour of cheapness.
RAID can be implemented either by dedicated hardware (RAID modules integrated into SCSI or SATA controller cards) or by software abstraction (the kernel). Whether hardware or software, a RAID system with enough redundancy can transparently stay operational when a disk fails; the upper layers of the stack (applications) can even keep accessing the data in spite of the failure. Of course, this “degraded mode” can have an impact on performance, and redundancy is reduced, so a further disk failure can lead to data loss. In practice, therefore, one will strive to only stay in this degraded mode for as long as it takes to replace the failed disk. Once the new disk is in place, the RAID system can reconstruct the required data so as to return to a safe mode. The applications won't notice anything, apart from potentially reduced access speed, while the array is in degraded mode or during the reconstruction phase.
When RAID is implemented by hardware, its configuration generally happens within the BIOS setup tool, and the kernel will consider a RAID array as a single disk, which will work as a standard physical disk, although the device name may be different (depending on the driver).
当 RAID由硬件实现时, RAID配置通常使用在BIOS设置工具中进行, 内核会把RAID阵列当成一个单独的磁盘对待, 根据驱动的不同设备名称可能会有所不同,但其使用方式和标准物理磁盘一样。



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