On 2015-05-20 21:08, Cejka Rudolf wrote:
> Dimitri Maziuk wrote (2015/05/20):

>> 160 < 180.
>>
>> you get it now?
>
> It should be "min. 160 < max. 180", which is false.

Actually, no: "up to" 160 so max. vs max.

> Tape users usually do not want to switch off hardware tape drive
> compression and use slower/faster better/worse software compression
> instead of it.

Hence comparing to write to zfs with compression on: 340MB/s observed to 
disk vs "up to 400MB/s" LTO sales pitch. (IRL you probably want to 
compress on the client before you push it over the network so 
compression on the storage becomes not so big a deal.)

...Also, max. disk speeds are just at the beginning of
> the disk.

That's the other part that ticks me off: don't compare the top of the 
line tape drive to a cheapest "desktop" drive. E.g. seagate's 15K 
"performance" line claims 240MB/s on the outside sectors and 140-ish 
(IIRC) on the inside: uncompressed head to platter. Yes, they do spell 
it out in that price bracket.

> And when the disk is not new, there could be very annoying speed
> instabilities due to internal sector reallocations and some other
> mysteries with no-so-bad/good sectors and heads.

It's called TLER and it's why you pay extra $20 for "nas" drive instead 
of the "desktop" one. But again, if you're comparing drive to tape: talk 
about when tape and drive aren't new: loose pins and worn motors that 
pull and stretch mylar and all that. Otherwise it's apples to oranges.

... I have seen small LBA
> interval slow downs even up to just 1 MB/s on one 2 TB drive, and the
> drive still reported very good SMART values - no reallocations, no
> errors.

Yes, I've thrown away a few of those. I've also thrown away a couple of 
LSI hardware raid cards (re your other post) recently and replaced them 
with basic "hba" controllers and zfs: flakey firmware, awful software, 
raids are not expandable, etc., etc. At this point my take on hardware 
raid is: "avoid like plague", if you need the speed add $5 and buy SSDs.

Dimitri


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