USB pendrive : the copy takes about 3 minutes but the unmount takes a very long time : 10 to 12 minutes
Solution 1
You are probably suffering from buffering caching. To speed up writing to USB sticks (and hard disks in general), Linux uses a filesystem cache:
When you (think you) write something to the stick then it is first written to the cache (in RAM) and the cp
command (for instance) returns immediately pretending a really fast write operation. While you do other things, the contents of the cache is then written to the stick in background. You may notice that an LED on the stick still flashes showing write operations (depends on your stick) although nothing apparent happens.
When you issue umount
soon after a write operation, then umount
waits until all the filesystem's cache content is written to the stick in order to make sure no data gets lost.
With sync
you can manually force emptying the cache and writing the data to the stick. However, this won't speed up the total elapsed time because then you will have to wait for the sync
to complete (instead of waiting for umount
). But the umount
will then return instantaneously because the cache is already flushed.
In summary you have three choices after copying large or many files to the stick:
umount
and wait 10 minutes for it to completesync
, wait 10 minutes to complete, followed byumount
(will return almost immediately)- simply wait for 10 minutes (perhaps a bit more) and do nothing (or something unrelated to the stick) and then issue
umount
. Because the cache gets written in background automatically,umount
will then return almost immediately as well.
Solution 2
When you copy files to your pendrive, they are not written on it directly. Filesystem synchronization is happening on unmount command, the actual data is written while you wait your unmount. If you execute sync
before umount
, the umount
is instant.
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SebMa
Updated on September 18, 2022Comments
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SebMa almost 2 years
I'm using Ubuntu 17.10.
I formatted an USB pen-drive to NTFS to prepare a Windows7 USB Installer.
I set the bootable flag on this pen-drive and copied the files into it.
EDIT 1: The USB pen-drive is automatically mounted by udev.
umount /dev/sdb1
takes from 10 upto 12 minutes to complete.Here are the mount options :
$ mount | grep sdb /dev/sdb1 on /media/mansfeld/Win7_USB_Installer type fuseblk (rw,nosuid,nodev,relatime,user_id=0,group_id=0,default_permissions,allow_other,blksize=4096,uhelper=udisks2)
EDIT 2: The
cp
operation is not INSTANTANEOUS at all, it took 3 minutes to copy the files to the USB pen-drive.EDIT 3: The
sync
operation (done right after thecp
) took 12 minutes to complete ! But then theumount
will be instantaneous.For FAT32, (with sync also disabled during mount), I notice the same behaviour.
Any ideas why it takes so long to unmount NTFS USB pendrive ?
-
sudodus over 6 yearsHow do you unmount it (which command or GUI action)? Did you try with the following command line?
sudo umount /dev/sdb1
; Maybe there are some buffered data, that must be written to the drive before it can be unmounted. You can flush the buffers withsync
; Aftersync
has returned to prompt, unmounting should be fast. -
SebMa over 6 years@sudodus The computer on which I ran this is at the university so I'll have to wait 'till monday to try and time the
sync
command. -
sudodus over 6 yearsI noticed your edit. This is according to my experience. Copying takes some time (at first it is fast (writing to RAM), but after a while, it must flush the buffer (when it is full or almost full according to some rule), and the copying will be slower). Yet, when the copy command finished (
cp
returns to prompt), there are still lots of data in the buffer. If you runsync
right after thecp
command, and wait untilsync
returns to prompt, the unmount should be fast. Please try that :-) -
sudodus over 6 yearsI think you know now :-)
-
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SebMa over 6 yearsThe computer on which I ran this is at the university so I'll have to wait 'till monday to try and time the
sync
command. -
Pasi Suominen over 6 yearsOr mount the filesystem with -o sync flag enabled, that disables write caching.
-
PerlDuck over 6 yearsYes, @Pasi, that's another reasonable approach. Thank you for pointing that out. But then the "10 minutes" will kick in when you actually
cp
the files. It just depends on what behaviour you prefer. Waiting forcp
or waiting forumount
. -
SebMa over 6 years@PasiSuominen How can I have
udev
mount my USB pen-drive automatically with thesync
option enabled ? -
Pasi Suominen over 6 years/etc/udev/rules.d/ is your answer. In udev rule you can specify mount parameters.
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SebMa over 6 years@PasiSuominen On ubuntu server, this dir is empty, what else can I do ?
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Pasi Suominen over 6 yearsYou can specify your device on /etc/fstab
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Owl almost 5 yearsI'll have to wait till monday for the sync to finish!