[Beowulf] Re: real hard drive failures

[Glen Gardner]

USB flash is really slow. Regular CF (@ 128 KB/s writes) on a cf to ide 
adapter is a lot faster (particularly write speed) than USB flash (@ 64 
KB/s write speed) "thumb drives".

I've had good luck with IBM microdrives, but CF is getting cheaper than 
microdrives.  Of course , the microdrives are a lot faster (@ 1MB/s R/W) 
than CF on write.  But CF is pretty fast on read (10 MB/s ??).

CF has a limited number of writes before it fails , anywhere from 100K 
to 1M write cycles. The time for write cycles is typically anywhere from 
300 milliseconds to 500 milliseconds for a 32 KB chunk for regular CF. 
 Typically you write a chunk of CF at once in each write cycle, and 32KB 
is a typical figure for that (but it varies with the particular memory 
chips used). This is why CF is so awfully slow when writing. Using 
serial CF makes it even worse, which is one reason why USB thumb drives 
are even slower than regular CF cards.

CF is okay for booting a system from, but things like /tmp , /var are 
best mountd in a memory file  system and only written to cf when 
shutting down.
Swap partitions and /home need to be mounted via NFS.  

At present, I have two nodes of a 14 node cluster booting from CF, and 
/home is mounted on another machine with a proper hard drive via NFS. 
 Ten of the nodes are booting from microdrives, and two nodes have ata 
133 hard drives for /home, development and backups.

/var  /tmp and swap are actually mounted on the cf card, and I'm waiting 
to see  how long before the cf actually expires.  These nodes have been 
up 24/7 for over a month now, with no problems. I have not tried to 
force the nodes to swap. For saving power and reducing heat, CF is going 
to be the best you can get. Microdrives are almost as good, laptop 
drives are pretty good, and a regular IDE drive is a pig in comparison.

I use a USB thumb drive with a bootable OS on it as an emergency boot 
drive.  It comes in handy when installing a node.
Since I use microdrives, all I do is shut down the node and plug the new 
microdrive into the cf adapter, and the cf thumb drive in the usb port 
and  turn the node on, and it boots from USB so I can then install a 
system image stored on the development node onto the new microdrive via 
an NFS mount. It takes about 5 minutes to install and configure a new 
node in this fashion. Writing the disk image to a 512 MB cf card is 
going to take up to an hour, and plan on at least twice that to write a 
disk image to a 512 MB USB flash. (CF is just plain slow)

Glen




Mark Hahn wrote:

>>>on that note, though - does anyone have comments about booting 
>>>machines from flash?
>>>
>>>      
>>>
>>I've booted a mini-ITX system from flash,
>>the distribution in question was a wireless access point.
>>All you need is a CF to IDE adapter.
>>    
>>
>
>I don't really see those much at all.  perhaps I'm not using 
>the right search terms.
>
>have you looked into booting from usb-flash?  that would be very 
>much dependent on bios, of course, but far more accessible.
>
>thanks, mark hahn.
>
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-- 
Glen E. Gardner, Jr.
AA8C
AMSAT MEMBER 10593
Glen.Gardner at verizon.net


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