Sorry for my ignorance if this question sounds stupid. Does performing an AV scan cause disk writes? Till now I had the idea that it only caused reads since the AV reads the files. Although I have startup scan disabled, I run a full scan with ESET once a month.
It's not a stupid question at all. I even had this question before. Usually, a scan does not cause much disk writes unless the AV has to scan an archive that has to be unpacked to scan or a big exe file that it determines to be suspicious, not whitelisted in the cloud and can be unpacked by the engine as well.
Usually smaller files are unpacked in memory to scan but bigger files are unpacked on disk.
For example, I had a cracked FIFA game a few years ago whose main cracked exe file was 500/600 MB+. ESET'S scanning engine internally determined it to be suspicious. The file size was way above the size that an AV typically submits to their cloud for deeper analysis or have it whitelisted. Such crack files are usually not whitelisted even though they are not malicious. Even Kaspersky's reputation was unknown even though it knew the age of the file and other info.
So after every signature update, this file was scanned by ESET. Often due to encryption or whatnot, such files cannot be unpacked by a scanning engine but this file was unpackable. This caused every ESET signature update to cause a total of 1.27 GB of disk writes.
This is why I said, based on what a user has on their PC, the disk writes by ESET can increase. In my ESET VM where not much is installed, it writes 530 MB or something. Btw, it's not like ESET scans every single crack game exe's. That was the only cracked exe I ever saw it scanning.
A monthly full scan is never required with something like ESET because it already scans after evey update the common locations where an active malware may persist. ESET can be configured to not scan archive, runtime packers and other things or startup scan itself can be completely turned off too but I won't recommend it.
Since it's a Bitdefender thread, Bitdefender due to their matured behavior blocker don't feel the need to scan files after every signature update.
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