Ubuntu – Copy to USB memory stick really slow

usb

When I copy files to the USB device, it takes much longer than in windows (same usb device, same port) it's faster than USB 1.0 speeds (1MB/s) but much slower than USB 2.0 speeds (12MB/s). To copy 1.8GB takes me over 10 minutes (it should be < 3 min.) I have two identical SanDisk Cruzer 8GB sticks, and I have the same problem with both. I have a super talent 32GB USB SSD in the neighboring port and it works at expected speeds.

The problem I seem to see in the GUI is that the progress bar goes to 90% almost instantly, completes to 100% a little slower and then hangs there for 10 minutes. Interrupting the copy at this point seems to result in corruption at the tail end of the file. If I wait for it to complete the copy is successful.

Any ideas? dmesg output below:

[64059.432309] usb 2-1.2: new high-speed USB device number 5 using ehci_hcd
[64059.526419] scsi8 : usb-storage 2-1.2:1.0
[64060.529071] scsi 8:0:0:0: Direct-Access     SanDisk  Cruzer           1.14 PQ: 0 ANSI: 2
[64060.530834] sd 8:0:0:0: Attached scsi generic sg4 type 0
[64060.531925] sd 8:0:0:0: [sdd] 15633408 512-byte logical blocks: (8.00 GB/7.45 GiB)
[64060.533419] sd 8:0:0:0: [sdd] Write Protect is off
[64060.533428] sd 8:0:0:0: [sdd] Mode Sense: 03 00 00 00
[64060.534319] sd 8:0:0:0: [sdd] No Caching mode page present
[64060.534327] sd 8:0:0:0: [sdd] Assuming drive cache: write through
[64060.537988] sd 8:0:0:0: [sdd] No Caching mode page present
[64060.537995] sd 8:0:0:0: [sdd] Assuming drive cache: write through
[64060.541290]  sdd: sdd1
[64060.544617] sd 8:0:0:0: [sdd] No Caching mode page present
[64060.544619] sd 8:0:0:0: [sdd] Assuming drive cache: write through
[64060.544621] sd 8:0:0:0: [sdd] Attached SCSI removable disk

Best Answer

Why is copying to my USB drive so slow in Linux (and faster in Windows)?

Reason 1. File caching can make writes appear slower or faster

The problem I seem to see in the GUI is that the progress bar goes to 90% almost instantly, completes to 100% a little slower and then hangs there for 10 minutes.

One thing you need to understand is file caching. Linux (and Windows) will use otherwise "empty" RAM to cache read/write operations and make them faster on subsequent accesses. Caching copy operations to slow devices results in the behavior you see -- the "fast completion" is actually writing to the cache, and then it slows and stops because the actual flushing of the data in the cache (sync) to the slow device is taking very long. If you abort at that point, the data is corrupted (as you noted) since the sync never finished.

Such copying in Windows may seem faster (including the reported MB/sec speeds) because sometimes Windows will not wait for the sync, and declare the job completed as soon as the data is written to cache.

Reason 2. Writing lots of files, especially small ones, is slow

To copy 1.8GB

Because of the way flash memory and filesystems work, the fastest throughput (speed) is achieved when writing very large files. Writing lots of small files, or even mixed data containing a number of small files can slow the process down a lot. This affects hard drives too, but to a somewhat lesser extent.

Reason 3. Write speeds of a USB stick and an SSD cannot be compared

I have a super talent 32GB USB SSD in the neighboring port and it works at expected speeds.

  • A garden variety USB stick usually consists of flash memory chips that are written to serially (sequentially), and does not have any cache of its own.

  • An SSD, on the other hand, contains a controller which writes to the flash memory chips parallel, increasing the throughput by a factor of 2 or more over the USB stick.

    • If your 32GB SSD had 4x 8GB chips, it would still be 4x faster than the USB stick at any write operation.
    • The SSD also contains RAM cache (like hard disks), so it can quickly store incoming data in the cache and tell the OS that it's done, while it still has to actually write that data to the flash memory.

  • So, with one large file, your 32GB GB with the 4x structure we assumed, would be 4x as fast; with many small files, it would be 10x or more faster because it could intelligently store them in its cache.

To sum up, these are the reasons why file copying to USB sticks may appear slower in Linux. Is it actually slower because of a hardware/driver issue or whatever....

Doing a proper comparison of write speeds between Linux and Windows

  • First of all, forget about the SSD because of reason 3. It's like oranges and apples.
  • To negate the effects of reason 1 (caching) and reason 2 (small files), you need to test with a single large file, larger than the amount of RAM on the test system.
  • In Linux you can create it with dd if=/dev/urandom of=largetest bs=1M count=7500, which gives you a 7500 MB test file. Assuming your system has less than 4GB or so of RAM, it's good enough. Copy that to a freshly formatted Sandisk 8GB stick, and time it.
  • Reboot in Windows, and copy largetest from the USB stick to your hard disk. Reboot again (to remove it from the cache). Then format the USB stick (same vfat/FAT32!), and copy largetest from the hard disk to the stick.
  • How do the times compare?
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