The Interleave Factor¶

If a ccd is interleaved correctly, a “striping” effect is achieved, which can increase sequential read/write performance. The interleave factor is expressed in units of DEV_BSIZE (usually 512 bytes). For large writes, the optimum interleave factor is typically the size of a track, while for large reads, it is about a quarter of a track. (Note that this changes greatly depending on the number and speed of disks.) For instance, with eight 7,200 RPM drives on two Fast-Wide SCSI buses, this translates to about 128 for writes and 32 for reads. A larger interleave tends to work better when the disk is taking a multitasking load by localizing the file I/O from any given process onto a single disk. You lose sequential performance when you do this, but sequential performance is not usually an issue with a multitasking load.

An interleave factor must be specified when using a mirroring configuration, even when you have only two disks (i.e., the layout winds up being the same no matter what the interleave factor). The interleave factor will determine how I/O is broken up, however, and a value 128 or greater is recommended.

ccd has an option for a parity disk, but does not currently implement it.

The best performance is achieved if all component disks have the same geometry and size. Optimum striping cannot occur with different disk types.

For random-access oriented workloads, such as news servers, a larger interleave factor (e.g., 65,536) is more desirable. Note that there is not much ccd can do to speed up applications that are seek-time limited. Larger interleave factors will at least reduce the chance of having to seek two disk-heads to read one directory or a file.

Disk Mirroring¶

You can configure the ccd to “mirror” any even number of disks. See ccdconfig(8) for how to specify the necessary flags. For example, if you have a ccd configuration specifying four disks, the first two disks will be mirrored with the second two disks. A write will be run to both sides of the mirror. A read will be run to either side of the mirror depending on what the driver believes to be most optimal. If the read fails, the driver will automatically attempt to read the same sector from the other side of the mirror. Currently ccd uses a dual seek zone model to optimize reads for a multi-tasking load rather than a sequential load.

In an event of a disk failure, you can use dd(1) to recover the failed disk.

Note that a one-disk ccd is not the same as the original partition. In particular, this means if you have a file system on a two-disk mirrored ccd and one of the disks fail, you cannot mount and use the remaining partition as itself; you have to configure it as a one-disk ccd. You cannot replace a disk in a mirrored ccd partition without first backing up the partition, then replacing the disk, then restoring the partition.

Linux Compatibility¶

The Linux compatibility mode does not try to read the label that Linux' md(4) driver leaves on the raw devices. You will have to give the order of devices and the interleave factor on your own. When in Linux compatibility mode, ccd will convert the interleave factor from Linux terminology. That means you give the same interleave factor that you gave as chunk size in Linux.

If you have a Linux md(4) device in “legacy” mode, do not use the CCDF_LINUX flag in ccdconfig(8). Use the CCDF_NO_OFFSET flag instead. In that case you have to convert the interleave factor on your own, usually it is Linux' chunk size multiplied by two.

Using a Linux RAID this way is potentially dangerous and can destroy the data in there. Since FreeBSD does not read the label used by Linux, changes in Linux might invalidate the compatibility layer.

However, using this is reasonably safe if you test the compatibility before mounting a RAID read-write for the first time. Just using ccdconfig(8) without mounting does not write anything to the Linux RAID. Then you do a fsck.ext2fs (ports/sysutils/e2fsprogs) on the ccd device using the -n flag. You can mount the file system read-only to check files in there. If all this works, it is unlikely that there is a problem with ccd. Keep in mind that even when the Linux compatibility mode in ccd is working correctly, bugs in FreeBSD's ex2fs implementation would still destroy your data.