Kernel Korner - Storage Improvements for 2.6 and 2.7
Storage has changed rapidly during the last decade. Prior to that, server-class disks were proprietary in all senses of the word. They used proprietary protocols, they generally were sold by the server vendor and a given server generally owned its disks, with shared-disk systems being few and far between.
When SCSI moved up from PCs to mid-range servers in the mid 1990s, things opened up a bit. The SCSI standard permitted multiple initiators (servers) to share targets (disks). If you carefully chose compatible SCSI components and did a lot of stress testing, you could build a shared SCSI disk cluster. Many such clusters were used in datacenter production in the 1990s, and some persist today.
One also had to be careful not to exceed the 25-meter SCSI-bus length limit, particularly when building three- and four-node clusters. Of course, the penalty for exceeding the length is not a deterministic oops but flaky disk I/O. This limitation required that disks be interspersed among the servers.
The advent of FibreChannel (FC) in the mid-to-late 1990s improved this situation considerably. Although compatibility was and to some extent still is a problem, the multi-kilometer FC lengths greatly simplified datacenter layout. In addition, most of the FC-connected RAID arrays export logical units (LUNs) that can, for example, be striped or mirrored across the underlying physical disks, simplifying storage administration. Furthermore, FC RAID arrays provide LUN masking and FC switches provide zoning, both of which allow controlled disk sharing. Figure 1 illustrates an example in which server A is permitted to access disks 1 and 2 and server B is permitted to access disks 2 and 3. Disks 1 and 3 are private, while disk 2 is shared, with the zones indicated by the grey rectangles.
This controlled sharing makes block-structured centralized storage much more attractive. This in turn permits distributed filesystems to provide the same semantics as do local filesystems, while still providing reasonable performance.
Modern inexpensive disks and servers have reduced greatly the cost of large server farms. Properly backing up each server can be time consuming, however, and keeping up with disk failures can be a challenge. The need for backup motivates centralizing data, so that disks physically located on each server need not be backed up. Backups then can be performed at the central location.
The centralized data might be stored on an NFS server. This is a reasonable approach, one that is useful in many cases, especially as NFS v4 goes mainstream. However, servers sometimes need direct block-level access to their data:
A given server may need a specific filesystem's features, such as ACLs, extended attributes or logging.
A particular application may need better performance or robustness than protocols such as NFS can provide.
Some applications may require local filesystem semantics.
In some cases, it may be easier to migrate from local disks to RAID arrays.
However, the 2.4 Linux kernel presents some challenges in working with large RAID arrays, including storage reconfiguration, multipath I/O, support for large LUNs and support for large numbers of LUNs. The 2.6 kernel promises to help in many of these areas, although there are some areas of improvement left for the 2.7 development effort.
Because most RAID arrays allow LUNs to be created, removed and resized dynamically, it is important that the Linux kernel to react to these actions, preferably without a reboot. The Linux 2.6 kernel permits this by way of the /sys filesystem, which replaced the earlier /proc interfaces. For example, the following command causes the kernel to forget about the LUN on busid 3, channel 0, target 7 and LUN 1:
echo "1" > \ /sys/class/scsi_host/host3/device/3:0:7:1/delete
The busid of 3 is redundant with the 3 in host3. This format also is used, however, in contexts where the busid is required, such as in /sys/bus/scsi/devices.
To later restore only that particular LUN, execute:
echo "0 7 1" > /sys/class/scsi_host/host3/scan
To resize this same LUN, use:
echo 1 > /sys/bus/scsi/devices/3:0:7:1/rescan
To scan all channels, targets and LUNs, try:
echo "- - -" > /sys/class/scsi_host/host3/scan