Apple Event: May 7th at 7 am PT
I do network file transfers from my M1 iMac (Monterey) to my old (late 2011) MacBook Pro running High Sierra. My router/internet is Google fiber, which is hugely fast on both machines. But these file transfers take AGES. Several minutes for 100MB. Why? I'd be better off walking back and forth with a USB stick.
iMac (24-inch, M1, 2021, 2 ports)
<< Are you saying that with network transfers to a slow rotating drive, speeds of only 1 MB/s are actually achievable? >>
I am saying my simple experiments showed that random transfers to a rotating magnetic drive (such as those performed by Finder doing file transfers) may be far slower than anyone expected. (You can download that same test software, free, and test your own drives.)
Sequential transfers are the fastest possible. They occur when ALL reads or writes are to the same area of the drive, and it can read and write complete disk tracks 'all at once'. No intervening reads or writes are occurring to move the drive heads away while it does these big blasts of reading and writing.
Random reads and writes are far more typical, where a few blocks are read or written, then something else occurs that repositions the drive heads, then it may go back or read or write to a different area of the drive. This type of use adds all the worst delays of using a Rotating magnetic drive:
Seek times -- to position the heads on the right track to get ready for a read or write.
Rotational latencies -- based on a typical half-spin of the platters delay for each read or write, strongly a function of platter spin rate.
<< Are you saying that with network transfers to a slow rotating drive, speeds of only 1 MB/s are actually achievable? >>
I am saying my simple experiments showed that random transfers to a rotating magnetic drive (such as those performed by Finder doing file transfers) may be far slower than anyone expected. (You can download that same test software, free, and test your own drives.)
Sequential transfers are the fastest possible. They occur when ALL reads or writes are to the same area of the drive, and it can read and write complete disk tracks 'all at once'. No intervening reads or writes are occurring to move the drive heads away while it does these big blasts of reading and writing.
Random reads and writes are far more typical, where a few blocks are read or written, then something else occurs that repositions the drive heads, then it may go back or read or write to a different area of the drive. This type of use adds all the worst delays of using a Rotating magnetic drive:
Seek times -- to position the heads on the right track to get ready for a read or write.
Rotational latencies -- based on a typical half-spin of the platters delay for each read or write, strongly a function of platter spin rate.
You sent me off to investigate what real-world speeds disk transfers might proceed at.
I used the free AmorphousDiskMark testing software on my 'slowest' Rotating Magnetic drive, (in its prime, one of the fastest such drives available) which is a WD Velociraptor 80 GB 7200 RPM, thought to be a very fast drive.
Sequential read and write speeds were just over 100 M Bytes/sec, Excellent. Then it did the random tests. WOW are they slower!
The QD references are Queue depth -- the number of commands Queued up to be executed, allowing the drive to execute out-of-order if its controller decides a different execution order is better.
The speed of your connection to the Internet has no bearing on this. Your local computer-to-computer transfers use only the Network "Switch' portion of your Router to perform a local transfer. Direct computer-to-computer.
Local transfers can proceed no faster than the speed of the slowest part of the network. This is likely to be the Wi-Fi to the older Mac. That is 802.11n with a top speed of at best 450 M bits/sec under absolutely IDEAL conditions. Typically more like 216 M bits/sec.
That 2011 MacBook Pro has a Gigabit Ethernet port. If you wired it to your Router, the transfer speed across that part of your network would improve to up to Gigabit speeds.
Thank you. That makes sense. I referred to Google Fiber because that's the router that we're talking about. But even at 200 Mb/s, my 100MB transfer should have happened in just a few seconds. 100MB in several minutes is just a few Mb/s. When connected to the internet, my older Mac pulls down internet data, through that same router, at 60 Mb/s, which would be 100 MB/13 seconds. My iMac does 900Mb/s. So why is my old Mac pulling data from my iMac, through the same router and network, at 100MB/several minutes?
Are these Rotating Magnetic drives at each end? Fastest speeds there are around 40 M Bytes/sec for Random access, and that is a super fast drive under ideal conditions. 20 M Bytes./sec is more typical for notebook and iMac computers using Rotating drives.
The whole business can operate no more quickly than the SLOWEST item in the chain.
I'm sorry, I'm having a hard time relating this info to my original question. Are you saying that with network transfers to a slow rotating drive, speeds of only 1 MB/s are actually achievable? Again, my old MBP, with a slow rotating drive, does internet downloads at 8-ish MB/s. But maybe that's sequential, rather than random? Not sure I understand the difference between sequential and random transfers.
In principle, this is an excellent reason to swap out an old rotating drive with an SSD in my MBP. I was figuring that boot time was the only reason to do that.
OK, so you're saying that a file transfer over a network is a random r/w, and maybe a speed test download from the internet is sequential? That would make sense. Yes, the speed of random r/w on rotational media is apparently appallingly slow. Of course, by that measure, just about any file manipulation activity on a machine will be appallingly slow with rotational media compared with an SSD. I had no idea.
<< In principle, this is an excellent reason to swap out an old rotating drive with an SSD in my MBP. >>
That is for sure!
on your older Mac, replacing the drive cable at the same time is strongly recommended as "preventatative maintenance"
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Random reads and writes is what MUST be used when the file size is unknown, such as Finder file transfers.
It is possible that the download tests create files of the right size, and can proceed with sequential reads and writes.
The old MBP does have rotating drives, and yes, that certainly will slow things down. The M1 iMac has SSD. But that still doesn't account for 100 MB/several minutes network transfer speed.
Excellent. Many thanks for your insights.
deathly slow network transfers
