RE: TM backup speeds...
I recently received an error from my Time Machine backup saying that I needed to reformat and erase my external hard drive before Time Machine could back up to the hard drive.
I followed the instruction and wiped my backup clean and reformatted. I then set up Time Machine backup again using the same hard drive but was curious about the estimated completion time. I have about 500 gb of data to backup and am using USB 2.0. Time Machine is estimating that it is going to take "about 10 days" to backup!!
That can't be right can it?? It has been backing up with a few hours now and I am only at 7 gb currently.
Is there something wrong with my hard drive or does this sound about right??
Hi downingp,
We can estimate a lower bound on the time it should take by using the maximum speed of a USB2.0 connection. This maximum communication speed is 480Mb/s (=60MB/s). Of course, the actual data transfer rate will be less than this because of overhead, etc., but we will be computing a lower bound on the time so we'll assume this maximum rate of 60MB/s.
Since you have 500GB to back up which is equal to 500000MB, then the time to backup this is 500000MB/(60MB/s)=8333.3 seconds=138.9 minutes or roughly 2 hours and 20 minutes. Mind you, this is the absolute minimum time for your backup. But this is so much lower than your "10 days estimate" that let's compute how fast you are backing up if indeed it takes 10 days=240 hours=14400 minutes=864000 seconds. Your data transfer rate would then be 500000MB/864000s=0.5787MB/s=578.7kB/s. This is considerably less than your theoretical maximum speed of 60MB/s, being over 100 times slower.
So, what is going on here? Well, there could be two possibilities.
First of all, when backups initially start they typically have to transfer many many tiny files, and because of the per file overhead, these tiny files are transferred at much reduced data rates. The "estimated backup times" are initially based upon the initially transferred tiny files, and so the time is way overestimated. Usually, even though the backup time is estimated at 10 days, as the backup continues and larger files are copied to the backup drive, the estimated times will decrease drastically. Backing up 500 GB over USB2.0 won't happen in the theoretically lower bound of 2h20m, but I would suspect it to happen in around 2 to 5 times this interval, so from 5 to 12 hours. If it is not done within a day, then I'd begin to worry.
Now let me ask, did you "erase" the free space on your drive when you reformatted it? Simply reformatting your problem drive may not have been enough. Why? Well, when a disk drive has read/write errors, it automatically moves the "blocks" having the R/W errors to unused saved "blocks" whose purpose is exactly this, to serve as a backup for blocks experiencing errors. The blocks having errors are then abandoned and the "extra" blocks are now used for all future storage. On a disk drive experiencing R/W errors, an "erase of the free space" will go through all of the blocks on the newly formatted drive and mark any that are having problems so that these marked blocks will not be used by the drive for its data storage. I'm not certain exactly what the Mac OS's Disk Utility formatting does, but it may very well require the "erase of free space" in order to mark all blocks having errors that should not be used. Now if you did not "erase free space" when you reformatted your USB2.0 HDD, then TM may be running into some "bad" blocks when it is backing up and any R/W errors may have a time-out of up to 7 seconds before the block is transferred to a new unused block. All of this can slow the TM backup.
(I also dont' know if TM actually tests the disk blocks before it backs up to them, it could but I sort of doubt it does, or the drive itself through ERC may be delaying the backup or the CoreStorage extensions of the OS may delay the backup when R/W errors are encountered.)
If you find that your USB2.0 HDD is having a lot of R/W errors, then it would be wise to abandon that drive as a backup. You risk loosing your backup if you don't. Personally, I backup to enterprise-class HDDs instead of commercial grade HDDs, as these have better warrantees and longer MTBFs, and thus hopefully they are better made with better tolerances and so should last longer. They also have better vibration isolation so are less prone to vibration induced troubles.
Regards,
Switon
