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I've noticed one of our web servers has incredibly slow disk drive speed when copying files. The server is a bare metal Dell server hosting a single VM (virtualized mainly for backup and DR purposes).

The VM has two VHD drives, one hosted on an SSD the other hosted on an SCSI HDD. The SSD based one is fine and performs at close to host speed. However even the read speed of the HDD based VHD is terrible. I'm only getting around 3MB per second when copying to the SSD and often it drops to 300KB per second when copying lots of smaller files. What could be causing this extreme loss in performance? The drive appears to have negligible fragmentation.

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    Well you are copying ~100k small files (about 35KB) and depending on the distribution of said files: HDDs ususally suck. Commented Feb 13, 2019 at 12:54
  • You did not mention the critical information: what VM hosts everything ? Commented Feb 13, 2019 at 13:45

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often it drops to 300KB per second when copying lots of smaller files.

Copy operations are serialized. New files are a lot of overhead to generate the file frst in the file system. Copying LOTS of small files in one thread, thus, means most time is spent waiting for the metadata operation, as well as opening etc. This is quite normal. Especialy on a HDD where the latency will make all file creation and operning a little tricky.

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