On 07/11/2014 06:12 PM, Konstantin Olchanski wrote:
On Fri, Jul 11, 2014 at 04:02:09PM -0400, Andrew Z wrote:
... synchronizing a flashing drive (target) with my hard drive (source) ...
Problem: it is slow -- takes three hours. To help the
speed issue, I upgraded from USB 2 to USB 3. Backup went
from 3 hr-15 min to 3 hr-5 min. It is almost faster
to wipe the stick and rewrite it.
The main question is this: what is the actual write speed of your USB flash
media? How about the re-write speed?
(not the same, obviously - as it requires as erase step).
Kanguru SS3: Read 105 MB/sec; 78 MB/sec
Don't have the re-write speed.
I ask because I use USB flash media as boot and linux system disks on embedded
machines (VME SBCs)
and I have looked at different USB flash media. Most of them are very slow,
actually it is very hard
to find "fast" USB flash media.
The cheap ones crawl. If you spend a little:
El-Cheap-O:
Read: 3 MB/sec
Write: 2 MB/sec
Kanguru SS3:
Read: 105 MB/sec
Write: 78 MB/sec
Kanguru Flash Blu 30:
Read: 145 MB/sec
Write: 8GB: 25 MB/sec
16,32,64GB: 45 MB/sec
Kingston Data Travler:
Read: 70 MB/sec
Write: 30 MB/sec
Kingston Data Travler Hyper-X 3.0:
Read: 225 MB/sec
Write: 135 MB/sec
For comparison: IntelSSD 530 Series SATA 3 flash drives:
Sequential Read: 540 MB/s
Sequential Write: 490 MB/s
The common media you get for $10 at Staples is "read 30M/s, write 10M/s"
(regardless of USB2 or USB3 interface).
Worse than that!
This is probably consistent with the speeds that you see.
I went from 20 MB/sec to 78 MB/sec. Took 10 minutes off of
three hours.
With some work, you can find media that writes at 20-30M/s, as measured by timing
"dd", but drops
severely when you time "rsync" (must be inefficient at writing small files).
So when you select a brand USB flash drive for your workload, as you run
"rsync", watch
the output of "vmstat 1" (the "bo" column is Mbytes/sec written to disk) and
the output of "iostat -x 1" - you will see %util pegged at 100% and "svctm" (in
msec) running
in 1-10-20 seconds for "slow media", a little bit smaller for betyter media.
For HDDs and SSDs,
the "svctm" is in low milliseconds. "svctm" is the request service time - time
from sending
a request to the drive and getting the reply from the drive that the request is
finished.
Some USB drives advertize high write speeds (not "up to" but actual "will write
at" promises),
you can try those ($$$), but you will probably find that the speed of "rsync"
does not reach
the promised rates because of inefficiency of flashing small files.
sda is the source hard drive; sdc is the target flash drive
$ iostat -x 1
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s
avgrq-sz avgqu-sz await svctm %util
sda 0.00 0.00 28.00 0.00 7168.00 0.00
256.00 0.08 2.79 1.39 3.90
sdc 0.00 0.00 0.00 280.00 0.00 7406.00
26.45 1.34 4.74 3.25 90.90
Couldn't find "svctm".
Does the above tell you anything?
P.S. Another problem with USB flash drives - all brands except for 1 or 2 do
not survive
being used as linux system disks - they brick themselves within days or weeks.
I notice
that they tend to run quite hot, so I suspect they simply overheat and die.
Actually,
I did not find a single USB3 flash drive that survives use as linux system disk
yet.
By luck I have enough Patriot RageXT 8GB and 16GB USB2 flash media, these seem
to last.
There is a lot of trash out there. I refuse to sell Buffalo,
as have had almost 100% returns on them.
--
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Computers are like air conditioners.
They malfunction when you open windows
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