> In order to use Notmuch, you will need to have your email messages stored in your local filesystem, one message per file.

One message per file? That's frustrating.

Doesn't really matter for ext4 drives, but I still have some ext3 ones, and past 10,000 files in a directory, performance really starts to suck.

What kernel are you running? The last kernel to use the ext3 driver to mount ext3 volumes by default is no longer maintained, as far as I'm aware. The ext3 driver was completely removed for 4.3. All kernels since 2.6.28 can mount ext2 with the ext4 driver, and I think all kernels with ext4 support can mount ext3 with the ext4 driver.

If you're using the ext4 driver, you can enable the dir_index feature (which I think is what you're looking for) with tune2fs.

You can do a proper conversion in a relatively short time, if it's a problem with the on-disk format. Pretty sure that ext4 is mainly just ext3 with journaling and a generally much better implementation.

A change to ext4 would be quite painless, but I'm not allowed to manage the drives on my work machine, which is unfortunate.

Out of interest, is there a reason why?

I get not wanting to bother with migrating old drives full of data but I would assume that emails are to be stored on your main system(s) with more current file systems.

ext3 on my work machine. Something I'm not allowed to change. (Hopefully the situation will change somewhere in the next 12 months).