Unfortunately no one can be told what fun_plug is - you have to see it for yourself.
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I have a DNS-325 with firmware v1.04 and ffp v0.7. There are two WD Red NAS 3TB drives in it, formatted as separate volumes.
Recently I noticed that write performance for big files on sdb decreased significantly, to nearly unusable.
I can write a big file to sda at >200 Mbps (as reported by NetSpeedMonitor on my PC), but when writing to sdb, speed quickly decreases to about 1-2 Mbps. Top shows >90% iowait on the NAS during the write operation. Small files are written at the same fast speed as to sda.
I checked SMART attributes using smartctl and all are normal. I also ran fsck (after killing smbd and unmounting sdb2). No problems there.
What else should I check? Also, is there a way to fsck sda2?
I decided to reformat the volume. First I deleted files that are backed up somewhere else. Now there is about 500 GB free space on the volume and write speed is fine again. At the time I wrote the previous post there was about 100 GB free space.
Perhaps it is normal behavior in case of diminishing free space? The file system is ext4 while on the other drive it is ext2.