Question to a developer ?

Question

Hello,
As many people, I try to prevent the (programmed) death of my external hard drives by allowing them to stop spinning when they are not required (in system preferences > Energy saver > put the hard disk(s) to sleep when possible.
This is a nice work around the fragility and crappy components used in today's hardware.
But this also the best way to drive you out of your mind, for example when you are simply browsing in Safari, and all in a sudden, Mac OS decides it absolutely needs to check something that is on one of your idling external disks, and not only you don't understand why it would have anything to do with these drives, but also you get stuck waiting there until your drive finishes to start up again before being able to continue browsing. This doesn't happen only in safari : Mac OS seems to have been designed to "randomly" need to use your external drives even if there is no apparent reason for it to need them. Sometimes the drives even start up one AFTER the other, making the unnecessary waste of time even more enjoyable!
I am sure everyone can agree the decision to have this work this way, was not taken on a very productive meeting in Apple Developement seminaire in the Caribbean.
So I would really enjoy an answer to :
• why would my computer need to access an external hard drive to load a page in a browser (it works fine when my drives are not plugged in you know..)
• why did a developer team think it would be a good idea to simply BLOCK all possible user activity, and just leave him hang there until the drive he never even asked for in the first place finishes spinning back up
Peace

Answer 1

Since this is a user forum and there aren't any Apple employees, I'll respond, but from the standpoint of being a sometimes Linux kernel hacker...
First, spinning down drives isn't hedge against hardware failure, but an energy-saving feature. Truth is that spinning down drives doesn't impact hardware life. When drives are spun down, there's clearly no mechanical wear, but the act of spinning them up or down produces more wear than leaving them spinning. In the end, the increased wear of spin up/down is offset by the reduced wear of the idle state -- meaning the setting has no net benefit (or detriment) with regard to wear on the drive. Energy-wise, though, the impact can be substantial.
With regard to drive access... UNIX systems in general, and including OS X, use a considerable amount of caching on disk I/O. Writes can queue for a while and may flush when caches fill or when the system idles, and reads will be from cache unless the cache is polluted or an object isn't in a cache. Web browsers in particular do astonishing amounts of I/O - each graphical element is a file, cookies are stored in a database, Javascript can have local storage, Flash can write temporary files, documents are cached, images and links are preloaded, etc.
That said, most of those cache misses will affect the boot-disk. If other drives (like externals) are spinning up, then it may be any number of causes. First, there's periodic tasks that run in the background that defragment files, refresh indexes, etc. If you have security software, it will periodically enumerate disks for scanning (waking everything up), and there are models of external drives (mostly cheap USB models) that require that they be polled periodically or they go to sleep entirely and disconnect.
Disk accesses are generally asynchronous, however, and should not affect the browser. The only reason that it would block is if the browser is making a request for disk enumeration. That would only happen if the page has Flash or Java content that requests it. Normal HTML wouldn't have that issue. Unfortunately, both Flash and Java (not JavaScript) do have features whereby they internally request and accounting of all the attached volumes. When they make that request, they block the browser until the function returns. You could always ask Adobe or Oracle to address that.