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SSD Behaviour, right or wrong?

post #1 of 7
Thread Starter 
Every day I make a backup of a 23GB Virtual Machine VHD file.

When I was pre-SSD, and I'd be copying it from one HDD to another HDD, it took 28 minutes.

I was very impressed when my boot drive switched to being SSD (an Intel X25-M G2 160GB) and that time (from SSD to HDD) dropped to 8 minutes.

I don't want or need to do this, I just wondered how much faster it would be, but just now I tried making a copy of that file on the same SSD. I expected to be blown away by the result - maybe it'd be 4 minutes, maybe even 3. Guess what though, the "estimated time remaining" showed 28 minutes. I got bored after a minute as it showed no sign of re-adjusting the time estimate down.

What's that all about? I'm surprised. I don't suspect it's a problem, and probably happens on any SSD, but what's the reason?

Note: I have two partitions on my SSD. I just tried copying a 800MB file from D: to D: and it took 22 seconds. Copying it from D: to C: took 11 seconds. I can't try copying the 23GB file from D: to C: as there's not enough room, but I suspect I'd be seeing that 8 minutes time again though.
    
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post #2 of 7
You mean your copying 'back' from your HDD to the SSD? If so, the HDD will be slow to read compared to the SSD. And the downside of the SSD will come into play in that it is slow to write (ie the speed is pretty much the same as a HDD).
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post #3 of 7
I believe that this is a case of sequential vs. random and the choking which this does in terms of SSD write performance. SSD to HD is simply a matter of reading the file from the flash and writing to the HD, in this case, the HD writes are the bottleneck.

copying SSD to itself sets up a random write-seek behavior as the drive is both calling for the next piece of the file to be read, as well as finding a place to write the file, doubling access time as well as dropping into the dreaded ssd random write speeds (25MB or less). This sounds like it would describe the slow copy times you're experiencing.
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post #4 of 7
Doing the calculations:
Size: 23GB
HDD to HDD: 14MB/s
SSD to HDD: 49MB/s

Size: 800MB
D to D: 36MB/s
D to C: 72MB/s

Intel X25-M 160GB specifications:
Sequential Read: up to 250MB/s
Sequential Write: up to 100MB/s
Random 4K Read (QD=32): up to 35K IOPS
Random 4K Write (QD=32): up to 8.6K IOPS

Thoughts:
  1. You must have some really old HDD's or at least some very fragmented ones. I normally get 60~100MB/s on HDD to HDD large file copies.
  2. Do you have AHCI enabled? That might help with the queuing. Otherwise, it's as ocpokey explained. Most likely, the reason why the D: to C: copy is faster is because the file is still in memory which makes it more a sequential write operation.
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post #5 of 7
Thread Starter 
Interesting. The HDD is probably quite fragmented, I haven't defragged it in at least a year (yeah I know, I'm bad). That 28 minutes would maybe drop to 24 if it was properly defragged.

As for the D: -> C: copy taking half the time of the D: -> D: copy when both of these partitions are on the same physical SSD, I don't quite get that still. After reading a block of data from D:, it then has to find an available sector to write to. Why would it make a difference whether it's D: or C: it's writing to? When it comes to hard drives and head positions and stuff, there's logic to that, but with a Flash drive, it's basically just one big hash table.
    
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post #6 of 7
Quote:
Originally Posted by d4005 View Post
As for the D: -> C: copy taking half the time of the D: -> D: copy when both of these partitions are on the same physical SSD, I don't quite get that still. After reading a block of data from D:, it then has to find an available sector to write to. Why would it make a difference whether it's D: or C: it's writing to?
Did you do the D: to D: copy first? If so, the file you copied may still be in the RAM hence, when you copied from D: to C:, the PC didn't have to hit the SSD as hard since it's no longer reading the data from the SSD but just writing the file from memory.
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post #7 of 7
Thread Starter 
Quote:
Originally Posted by rui-no-onna View Post
Did you do the D: to D: copy first? If so, the file you copied may still be in the RAM hence, when you copied from D: to C:, the PC didn't have to hit the SSD as hard since it's no longer reading the data from the SSD but just writing the file from memory.
Very good point. I'm in the middle of a massive copy operation right now, so trying it again would be unreliable. I'll try it properly in the morning.
    
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