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Old 12-28-2011, 11:00 AM
Sam Varshavchik
Default Implementing a "sometimes" RAID on a laptop. (eSATA, SSD, RAID 1)

linux guy writes:

I experienced a complete SSD failure this week on my laptop.

I've ordered a new Dell XPS 17 laptop which has an eSATA port.

I'm sure that Dell appreciates your business. When I had a hard drive fail
in one of my laptops, I simply replaced the hard drive.

Question 1.

How does one set up a "sometimes" RAID ?

One does not. RAID is permanent. If one disk fails, the array becomes
"degraded". If the failed disk is suitably replaced, RAID resyncs the data
onto the replacement, from the remaining working disk.

I suppose one can always manually fail a disk, then remove it from the
array. Then add it back to the array later. Then, wait a few hours while
RAID copies the entire disk.

Or would something like
rsync be better ? What happens the first time I plug my laptop into
the eSATA cable after being away from my desk ? What happens if there
is both new data and an error in existing data ? How does the RAID
software know the difference ?

RAID doesn't care. It duplicates disk blocks. It has no knowledge of the
actual data. Whether its extX, xFAT, swap partitions, or something else.
It's all just disk blocks, with some jumble of data on them, to RAID.

Question 2.

Internally, my XPS17 has 2 hard drives. I will probably use an SSD
for the OS and a 750 GB 7200 RPM conventional drive for data.

How does one configure the single external eSATA RAID drive to back up
(mirror) the data for both internal drives ?

If you want to set up RAID, when you install Fedora you'll need to choose
the custom partition layout. Create identically-sized partitions on both
disks, indicating that they're RAID partitions. Then take each pair of
partitions, one from each disk, and create a RAID volume on them, then
indicate what each volume is formatted as, and where it's mounted, as usual,
/, /boot, etc…

Question 3.

The OS drive will be an SSD which is faster than the eSATA RAID drive,
which will probably be a 7200 RPM 2TB+ conventional drive. Will this
limit the speed of the SSD to that of the eSATA drive or is buffering
employed to allow one to be faster than the other ?

The overall RAID speed will be limited to the slowest drive. There is some
buffering, and the system is not going to be allocating 100% of the CPU to
disk I/O, all the time. But, your max disk read/write throughput will, of
course, be limited to the slowest drive, and, for writing, the max write
speed gets dictated by the sum total performance of both drives, since each
disk block must be written to both drives.

Question 4.

Can only a portion of the eSATA RAID drive be allocated to the RAID
and the rest left to be mounted by the laptop for general access ?


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