Subject: Re: wedges and DEV_BSIZE (Was Re: removing VOPs)
To: Pavel Cahyna <pavel.cahyna@st.mff.cuni.cz>
From: Reinoud Zandijk <reinoud@netbsd.org>
List: tech-kern
Date: 10/11/2005 13:56:06
--jRHKVT23PllUwdXP
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Hi Pavel,
On Tue, Oct 11, 2005 at 10:39:16AM +0200, Pavel Cahyna wrote:
> Wouldn't it make sense if other drivers having larger sectors did the
> same? Why can't DVD/CD+-(M)R(W) do read-modify-write?
technically they can, though the penalties are huge.
1) switching from reading to writing and back take typically
takes significant time in the order of about *half a second* on a
switch.
2) you will loose sectors when writing on recordables. This results in a
few logical sectors NOT being adressable anymore. I dont have to tell
you what an administrative mess this gives on allocation.
3) CD-MRW and DVD+(M)RW are the only ones that allow you to write a *single
sector*. Others even require a whole ECC block to be written forcing a
read/modify/write cycle itself if not in cache so the filingsystem
implementation needs to be optimised to minimise this as much as
possible or performance will be very bad.
Cheers,
Reinoud
--jRHKVT23PllUwdXP
Content-Type: application/pgp-signature
Content-Disposition: inline
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.2.6 (NetBSD)
iQEVAwUBQ0uoT4KcNwBDyKpoAQLd4gf/SJ5JXWI+vD4mc4Zuq5BqoVoG3SxnncGe
1QEuOMxyFOxGcFZAXqiTgOb1WS4RIxUNA5Lx+6XLvbdfkIIcJiCey6kB+2cbEK4H
/kfBQvn/dI2+4boyIZYvwPoZ2N1W3NJdbFXQvROZhekGOGHRjfJTpUsw8yesCpF8
BMAunT0BmcFp5Yi+5VMMH/AhBqoJnOPUcGzp7SpHA++HbqaCBZFYBSDp0XsqTwJi
8CyyvX2WY5KUGt3abHe9obyGjKJGdewy7JILbux0rKwu4/B1zltGbKqSmKbHkFwi
NBO8R4PelgCH0915LSGqhPoS2s61v0MolGghM6fzd9Jkd60TUs/K/w==
=l6Pb
-----END PGP SIGNATURE-----
--jRHKVT23PllUwdXP--